Polymer membranes for continuous analyte sensors

ABSTRACT

Devices and methods are described for providing continuous measurement of an analyte concentration. In some embodiments, the devices include a membrane that has an interference domain designed to reduce the permeation of one or more interferents.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 13/594,734, filed Aug. 24, 2012, which is a continuation of U.S. application Ser. No. 13/594,602, filed Aug. 24, 2012, which claims the benefit of priority under 35 U.S.C. § 119(e) of U.S. Provisional Application No. 61/527,856 filed Aug. 26, 2011, the disclosure of each of which is hereby expressly incorporated by reference in its entirety and is hereby expressly made a portion of this application.

FIELD OF THE INVENTION

Devices and methods are described for providing continuous measurement of an analyte concentration. In some embodiments, the device has an interference domain that reduces permeation of one or more interferents into an electrochemically reactive surface. The interference domain can be configured to be more permeable or less permeable to one or more interferents than to a measured analyte species.

BACKGROUND OF THE INVENTION

Electrochemical sensors are useful in chemistry and medicine to determine the presence or concentration of a biological analyte. Such sensors are useful, for example, to monitor glucose in diabetic patients and lactate during critical care events. A variety of intravascular, transcutaneous and implantable sensors have been developed for continuously detecting and quantifying blood glucose values. Many implantable glucose sensors suffer from complications within the body and provide only short-term or less-than-accurate sensing of blood glucose. Similarly, many transcutaneous and intravascular sensors have problems in accurately sensing and reporting back glucose values continuously over extended periods of time, for example, due to noise on the signal caused by interfering species or unknown noise-causing events.

SUMMARY OF THE INVENTION

In the first aspect, an implantable device is provided for continuous measurement of an analyte concentration, the implantable device comprising: a sensor configured to generate a signal indicative of a concentration of an analyte in a host; and a membrane located over the sensor, the membrane comprising a first domain configured to reduce a flux therethrough of the analyte, a second domain that comprises an enzyme, and a third domain configured to reduce passage therethrough of an interferent, the third domain comprising a plurality of alternating polyelectrolyte layers.

In an embodiment of the first aspect, the plurality of alternating polyelectrolyte layers of the third domain comprises two types of layers. In some embodiments, the two types of layers comprise a polycationic layer and a polyanionic layer. In some embodiments, the polycationic layers form the most distal and the most proximal layers of the third domain. In some embodiments, the third domain comprises an odd number of alternating polyelectrolyte layers. In some embodiments, the third domain comprises at least three alternating polyelectrolyte layers. In some embodiments, the first and third most distal layers with respect to the sensor are polycationic layers, and the second most distal layer with respect to the sensor is a polyanionic layer. In some embodiments, the third domain comprises at least five alternating layers. In some embodiments, the first, third, and fifth most distal layers with respect to the sensor are polycationic layers, and the second and fourth most distal layers with respect to the sensor are polyanionic layers. In some embodiments, the third domain comprises at least seven alternating polyelectrolyte layers. In some embodiments, the first, third, fifth, and seventh most distal layers with respect to the sensor are polycationic layers, and the second, fourth, and sixth most distal layers with respect to the sensor are polyanionic layers. In some embodiments, the two types of layers comprise a first layer type configured to selectively reduce passage therethrough of a first interferent and a second layer type configured to selectively reduce passage therethrough of a second interferent.

In an embodiment of the first aspect, the plurality of alternating polyelectrolyte layers comprises at least three types of layers. In some embodiments, the at least three types of layers comprise a first layer or bilayer type configured to selectively reduce passage therethrough of a first interferent, a second layer bilayer type configured to selectively reduce passage therethrough of a second interferent, and a third layer bilayer type configured to selectively reduce passage therethrough of a third interferent.

In an embodiment of the first aspect, the first domain is proximal to the second domain with respect to the sensor, and wherein the second domain is proximal to the third domain with respect to the sensor.

In an embodiment of the first aspect, the sensor comprises an electroactive surface.

In a second aspect, an implantable device is provided for continuous measurement of an analyte concentration, the implantable device comprising: a sensor configured to generate a signal indicative of a concentration of an analyte in a host; and a membrane located over the sensor, the membrane comprising a first domain configured to reduce a flux therethrough of the analyte, a second domain that comprises an enzyme, and a third domain configured to reduce passage therethrough of an interferent, the third domain comprising an self-assembled, ordered structure formed from a plurality of alternating polyelectrolyte layers.

In an embodiment of the second aspect, the plurality of layers is formed of an acetylene-based polymer.

In an embodiment of the second aspect, the plurality of layers is cross-linked.

In an embodiment of the second aspect, the plurality of alternating polyelectrolyte layers comprises a polycationic layer and a polyanionic layer. In some embodiments, the polycationic layer is configured to selectively reduce passage therethrough of a first interferent and the polyanionic layer is configured to selectively reduce passage therethrough of a second interferent. In some embodiments, the polycationic layers form the most distal and the most proximal layers of the third domain. In some embodiments, the third domain comprises an odd number of alternating polyelectrolyte layers. In some embodiments, the third domain comprises at least three alternating layers. In some embodiments, the first and third most distal layers with respect to the sensor are polycationic layers, and the second most distal layer with respect to the sensor is a polyanionic layer. In some embodiments, the third domain comprises at least five alternating polyelectrolyte layers. In some embodiments, the first, third, and fifth most distal layers with respect to the sensor are polycationic layers, and the second and fourth most distal layers with respect to the sensor are polyanionic layers. In some embodiments, the third domain comprises at least seven alternating polyelectrolyte layers. In some embodiments, the first, third, fifth, and seventh most distal layers with respect to the sensor are polycationic layers, and the second, fourth, and sixth most distal layers with respect to the sensor are polyanionic layers.

In an embodiment of the second aspect, the third domain has an interferent-to-H₂O₂ sensitivity ratio of less than about 0.8 to about zero.

In an embodiment of the second aspect, the third domain has an interferent-to-H₂O₂ sensitivity ratio of less than about 0.6 to about zero.

In an embodiment of the second aspect, the third domain has an interferent-to-H₂O₂ sensitivity ratio of less than about 0.3 to about zero.

In an embodiment of the second aspect, the third domain has a thickness of from about 10 microns to about 10 nanometers.

In an embodiment of the second aspect, the third domain has a thickness of from about 500 nanometers to about 200 nanometers.

In a third aspect, a device for continuous measurement of glucose concentration is provided, the device comprising: an electrode configured to generate a signal indicative of a concentration of glucose in a host; and a membrane located over the electrode, the membrane comprising: a first domain that comprises an enzyme configured to react with glucose; a second domain configured to reduce passage therethrough of an interferent, the second domain comprising polyelectrolytes, whereby an equivalent peak glucose response to a 1,000 mg dose of the interferent administered to a host is less than about 100 mg/dL.

In an embodiment of the third aspect, the equivalent peak glucose response to a 1,000 mg dose of the interferent administered to a host is less than about 50 mg/dL.

In an embodiment of the third aspect, the equivalent peak glucose response to a 1,000 mg dose of the interferent administered to a host is less than about 25 mg/dL.

In an embodiment of the third aspect, the interferent can be acetaminophen.

In an embodiment of the third aspect, the interferent can be uric acid.

In an embodiment of the third aspect, the interferent can be ascorbic acid.

In an embodiment of the third aspect, the interferent is acetaminophen, and the second domain has an acetaminophen-to-H₂O₂ sensitivity ratio of less than about 0.8 to about zero.

In an embodiment of the third aspect, the interferent is acetaminophen, and the second domain has an acetaminophen-to-H₂O₂ sensitivity ratio of less than about 0.6 to about zero.

In an embodiment of the third aspect, the interferent is acetaminophen, and the second domain has an acetaminophen-to-H₂O₂ sensitivity ratio of less than about 0.3 to about zero.

In an embodiment of the third aspect, the interferent is uric acid, and the second domain has a uric acid-to-H₂O₂ sensitivity ratio of less than about 0.8 to about zero.

In an embodiment of the third aspect, the interferent is uric acid, and the second domain has a uric acid-to-H₂O₂ sensitivity ratio of less than about 0.6 to about zero.

In an embodiment of the third aspect, the interferent is uric acid, and the second domain has a uric acid-to-H₂O₂ sensitivity ratio of less than about 0.3 to about zero.

In an embodiment of the third aspect, the interferent is ascorbic acid, and the second domain has an ascorbic acid-to-H₂O₂ sensitivity ratio of less than 0.8 to about zero.

In an embodiment of the third aspect, the interferent is ascorbic acid, and the second domain has an ascorbic acid-to-H₂O₂ sensitivity ratio of less than about 0.6 to about zero.

In an embodiment of the third aspect, the interferent is ascorbic acid, and the second domain has an ascorbic acid-to-H₂O₂ sensitivity ratio of less than about 0.3 to about zero.

In an embodiment of the third aspect, the second domain comprises a plurality of alternating polycationic and polyanionic layers.

In a fourth aspect, a method is provided for forming a membrane on an implantable device configured for continuous measurement of an analyte concentration, comprising: applying a first solution to an implantable sensor or a layer deposited thereon, the first solution having a first pH and comprising polycations; drying the first solution to form a polycationic layer; applying a second solution to the polycationic layer, the second solution having a second pH and comprising polyanions; and drying the second solution to form a polyanionic layer.

In an embodiment of the fourth aspect, the first pH is approximately the pK_(b) of the polycations.

In an embodiment of the fourth aspect, the first pH is from about 9 to 10.

In an embodiment of the fourth aspect, the second pH is approximately the pK_(a) of the polyanions.

In an embodiment of the fourth aspect, the pH is from about 2 to 3.

In an embodiment of the fourth aspect, the method further comprises applying a solution comprising an enzyme onto at least one of a polycationic layer or a polyanionic layer to form an enzyme layer. In some embodiments, the analyte is glucose, and the enzyme is glucose oxidase.

In a fifth aspect, an implantable device is provided for continuous measurement of glucose concentration, the device comprising: an electrode configured to generate a signal indicative of a concentration of glucose in a host; and a membrane located over the electrode, the membrane comprising: a first domain that comprises an enzyme configured to react with glucose; and a second domain configured to reduce passage therethrough of an interferent, the second domain comprising a plurality of alternating polyelectrolyte layers, wherein the plurality of alternating polyelectrolyte layers comprise at least one of a polycationic layer and a polyanionic layer.

In an embodiment of the fifth aspect, the second domain comprises an odd number of alternating polyelectrolyte layers.

In an embodiment of the sixth aspect, the second domain comprises a plurality of polycationic layers, and wherein the polycationic layers form the most distal layer and the most proximal layer of the second domain.

In an embodiment of the fifth aspect, the second domain comprises at least three alternating polyelectrolyte layers. In some embodiments, the first and third most distal layers with respect to the electrode are polycationic layers, and the second most distal layer with respect to the electrode is a polyanionic layer.

In an embodiment of the fifth aspect, the second domain comprises at least five alternating polyelectrolyte layers. In some embodiments, the first, third, and fifth most distal layers with respect to the electrode are polycationic layers, and the second and fourth most distal layers with respect to the electrode are polyanionic layers.

In an embodiment of the fifth aspect, the second domain comprises at least seven alternating polyelectrolyte layers. In some embodiments, the first, third, fifth, and seventh most distal layers with respect to the electrode are polycationic layers, and the second, fourth, and sixth most distal layers with respect to the sensor are polyanionic layers.

In an embodiment of the fifth aspect, the first domain is distal to the second domain with respect to the electrode.

In an embodiment of the fifth aspect, the electrode comprises an electroactive surface. In some embodiments, the second domain contacts the electroactive surface.

In an embodiment of the fifth aspect, the polycationic layer comprises a polycation with an average linear charge density from about 2 to 10 e/Å, and the polyanionic layer comprises a polyanion with an average linear charge density from about 2 to 10 e/Å.

In an embodiment of the fifth aspect, the polycationic layer comprises a polycation with an average linear charge density from about 2 to 3 e/Å, and the polyanionic layer comprises a polyanion with an average linear charge density from about 2 to 3 e/Å.

In a sixth aspect, an implantable device is provided for continuous measurement of glucose concentration, the device comprising: an electrode configured to generate a signal indicative of a concentration of an glucose in a host; and a membrane located over the electrode, the membrane comprising: a first domain that comprises an enzyme configured to react with glucose; a second domain configured to reduce passage therethrough of an interferent, the second domain comprising a plurality of alternating polyelectrolyte layers, wherein the plurality of alternating polyelectrolyte layers comprise a first polycationic layer, a first polyanionic layer, and a second polycationic layer, wherein the first polycationic layer and the second polycationic layer have at least one characteristic that is different.

In an embodiment of the sixth aspect, the first polycationic layer is formed of a first material, the second polycationic layer is formed of a second material, and the first material is different from the second material.

In an embodiment of the sixth aspect, the first polycationic layer has a first average linear charge density, the second polycationic layer has a second average linear charge density, and the first average linear charge density is different than the second average linear charge density.

In an embodiment of the sixth aspect, the electrode comprises an electroactive surface. In some embodiments, the second domain contacts the electroactive surface.

In a seventh aspect, an implantable device is provided for continuous measurement of glucose concentration, the device comprising: an electrode configured to generate a signal indicative of a concentration of an glucose in a host; and a membrane located over the electrode, the membrane comprising: a first domain that comprises an enzyme configured to react with glucose; a second domain configured to reduce passage therethrough of an interferent, the second domain comprising a plurality of alternating polyelectrolyte layers, wherein the plurality of alternating polyelectrolyte layers comprise a first anionic layer, a first polycationic layer, and a second polyanionic layer, wherein the first polyanionic layer and the second polyanionic layer have at least one characteristic that is different.

In an embodiment of the seventh aspect, the first polyanionic layer is formed of a first material, the second polyanionic layer is formed of a second material, and the first material is different from the second material.

In an embodiment of the seventh aspect, the first polyanionic layer has a first average linear charge density, wherein the second polyanionic layer has a second average linear charge density, and wherein the first average linear charge density is different than the second average linear charge density.

In an embodiment of the seventh aspect, the electrode comprises an electroactive surface. In some embodiments, the second domain contacts the electroactive surface.

In an eighth aspect, an implantable device is provided for continuous measurement of glucose concentration, the device comprising: an electrode configured to generate a signal indicative of a concentration of glucose in a host; and a membrane located over the electrode, the membrane comprising: a first domain that comprises an enzyme configured to react with glucose and produce hydrogen peroxide; a second domain configured to reduce passage therethrough of an interferent, the second domain comprising a plurality of alternating polyelectrolyte layers, wherein the plurality of alternating polyelectrolyte layers comprise at least one of a polycationic layer and a polyanionic layer, whereby the device is configured to exhibit a selectivity for acetaminophen over hydrogen peroxide of less than about 0.5, wherein the selectivity is determined by dividing a sensitivity of the device for acetaminophen by a sensitivity of the device for hydrogen peroxide, wherein the acetaminophen sensitivity is determined in vitro by measuring the device's current response versus concentrations of acetaminophen at 10 μM aqueous acetaminophen, 50 μM aqueous acetaminophen, and 100 μM aqueous acetaminophen, and then performing linear regression, and wherein the hydrogen peroxide sensitivity is determined in vitro by measuring the device's current response versus concentrations of hydrogen peroxide at 1 μM aqueous hydrogen peroxide, 2 μM aqueous hydrogen peroxide, and 3 μM aqueous hydrogen peroxide, and then performing linear regression.

In an embodiment of the eighth aspect, the electrode comprises an electroactive surface. In some embodiments, the second domain contacts the electroactive surface.

In an embodiment of the eighth aspect, the membrane further comprises a third domain configured to reduce a flux therethrough of glucose.

In an embodiment of the eighth aspect, the device is configured to exhibit a selectivity for acetaminophen over hydrogen peroxide of less than about 0.047.

In an embodiment of the eighth aspect, the device is configured to exhibit a selectivity for acetaminophen over hydrogen peroxide of less than about 0.013.

In an embodiment of the eighth aspect, the device is configured to exhibit a selectivity for acetaminophen over hydrogen peroxide of less than about 0.006.

In an embodiment of the eighth aspect, the polycationic layer comprises a polycation with an average linear charge density from about 2 to 10 e/Å, and wherein the polyanionic layer comprises a polyanion with an average linear charge density from about 2 to 10 e/Å.

In an embodiment of the eighth aspect, the polycationic layer comprises a polycation with an average linear charge density from about 2 to 3 e/Å, and wherein the polyanionic layer comprises a polyanion with an average linear charge density from about 2 to 3 e/Å.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an expanded view of an exemplary embodiment of a continuous analyte sensor.

FIGS. 2A-2C are cross-sectional views through the sensor of FIG. 1 on line 2-2, illustrating various embodiments of the membrane system.

FIG. 3 is a graph illustrating the components of a signal measured by a glucose sensor (after sensor break-in was complete), in a non-diabetic volunteer host.

FIG. 4A is a schematic view of a base polymer containing surface-active end groups in one embodiment.

FIG. 4B is a schematic view of a bioprotective domain, showing an interface in a biological environment (e.g., interstitial space or vascular space).

FIG. 5A is a graph illustrating the H₂O₂ selectivity of polyelectrolyte coated sensors having from one to four layers versus a control.

FIG. 5B is a graph illustrating the interferent blocking properties of polyelectrolyte coated sensors having from one to four layers versus a control.

FIG. 6A is a graph illustrating the effect of PAA solution pH on acetaminophen selectivity (PAH solution held at a pH of 10.0).

FIG. 6B is a graph illustrating the effect of PAH solution pH on acetaminophen selectivity (PAA solution held at a pH of 3.0).

FIG. 7A is a graph illustrating the effect of PAA solution pH on acetaminophen selectivity.

FIG. 7B is a graph illustrating the effect of PAH solution pH on acetaminophen selectivity.

FIG. 8A is a graph comparing H₂O₂ and acetaminophen sensitivity for various interference domains in combination with an enzyme domain.

FIG. 8B is a graph illustrating the ratio of the sensitivity of acetaminophen to the sensitivity of H₂O₂.

FIG. 9A is a schematic view of a portion of one embodiment of an interference domain that comprises a plurality of polycationic and polyanionic layers.

FIG. 9B illustrates one embodiment of a layer-by-layer deposition method, which employs alternating adsorption of polycations and polyanions to create a structure illustrated in FIG. 9A

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following description and examples describe in detail some exemplary embodiments of devices and methods for providing continuous measurement of an analyte concentration. It should be appreciated that there are numerous variations and modifications of the devices and methods described herein that are encompassed by the present invention. Accordingly, the description of a certain exemplary embodiment should not be deemed to limit the scope of the present invention.

Definitions

In order to facilitate an understanding of the devices and methods described herein, a number of terms are defined below.

The term ‘analyte’ as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and is not to be limited to a special or customized meaning), and refers without limitation to a substance or chemical constituent in a biological fluid (for example, blood, interstitial fluid, cerebral spinal fluid, lymph fluid, urine, sweat, saliva, etc.) that can be analyzed. Analytes can include naturally occurring substances, artificial substances, metabolites, or reaction products. In some embodiments, the analyte for measurement by the sensing regions, devices, and methods is glucose. However, other analytes are contemplated as well, including, but not limited to: acarboxyprothrombin; acylcarnitine; adenine phosphoribosyl transferase; adenosine deaminase; albumin; alpha-fetoprotein; amino acid profiles (arginine (Krebs cycle), hi stidine/urocanic acid, homocysteine, phenylalanine/tyrosine, tryptophan); andrenostenedi one; antipyrine; arabinitol enantiomers; arginase; benzoylecgonine (cocaine); biotinidase; biopterin; c-reactive protein; carnitine; carnosinase; CD4; ceruloplasmin; chenodeoxycholic acid; chloroquine; cholesterol; cholinesterase; conjugated 1-β hydroxy-cholic acid; cortisol; creatine kinase; creatine kinase MM isoenzyme; cyclosporin A; d-penicillamine; de-ethyl chloroquine; dehydroepiandrosterone sulfate; DNA (acetylator polymorphism, alcohol dehydrogenase, alpha 1-antitrypsin, cystic fibrosis, Duchenne/Becker muscular dystrophy, glucose-6-phosphate dehydrogenase, hemoglobin A, hemoglobin S, hemoglobin C, hemoglobin D, hemoglobin E, hemoglobin F, D-Punjab, beta-thalassemia, hepatitis B virus, HCMV, HIV-1, HTLV-1, Leber hereditary optic neuropathy, MCAD, RNA, PKU, Plasmodium vivax, sexual differentiation, 21-deoxycortisol); desbutylhalofantrine; dihydropteridine reductase; diptheria/tetanus antitoxin; erythrocyte arginase; erythrocyte protoporphyrin; esterase D; fatty acids/acylglycines; free β-human chorionic gonadotropin; free erythrocyte porphyrin; free thyroxine (FT4); free tri-iodothyronine (FT3); fumarylacetoacetase; galactose/gal-1-phosphate; galactose-1-phosphate uridyltransferase; gentamicin; glucose-6-phosphate dehydrogenase; glutathione; glutathione perioxidase; glycocholic acid; glycosylated hemoglobin; halofantrine; hemoglobin variants; hexosaminidase A; human erythrocyte carbonic anhydrase I; 17-alpha-hydroxyprogesterone; hypoxanthine phosphoribosyl transferase; immunoreactive trypsin; lactate; lead; lipoproteins ((a), B/A-1, β); lysozyme; mefloquine; netilmicin; phenobarbitone; phenytoin; phytanic/pristanic acid; progesterone; prolactin; prolidase; purine nucleoside phosphorylase; quinine; reverse tri-iodothyronine (rT3); selenium; serum pancreatic lipase; sissomicin; somatomedin C; specific antibodies (adenovirus, anti-nuclear antibody, anti-zeta antibody, arbovirus, Aujeszky' s disease virus, dengue virus, Dracunculus medinensis, Echinococcus granulosus, Entamoeba histolytica, enterovirus, Giardia duodenalisa, Helicobacter pylori, hepatitis B virus, herpes virus, HIV-1, IgE (atopic disease), influenza virus, Leishmania donovani, Leptospira, measles/mumps/rubella, Mycobacterium leprae, Mycoplasma pneumoniae, Myoglobin, Onchocerca volvulus, parainfluenza virus, Plasmodium falciparum, poliovirus, Pseudomonas aeruginosa, respiratory syncytial virus, rickettsia (scrub typhus), Schistosoma mansoni, Toxoplasma gondii, Trepenoma pallidium, Trypanosoma cruzi/rangeli, vesicular stomatis virus, Wuchereria bancrofti, yellow fever virus); specific antigens (hepatitis B virus, HIV-1); succinylacetone; sulfadoxine; theophylline; thyrotropin (TSH); thyroxine (T4); thyroxine-binding globulin; trace elements; transferrin; UDP-galactose-4-epimerase; urea; uroporphyrinogen I synthase; vitamin A; white blood cells; and zinc protoporphyrin. Salts, sugar, protein, fat, vitamins, and hormones naturally occurring in blood or interstitial fluids can also constitute analytes in certain embodiments. The analyte can be naturally present in the biological fluid or endogenous, for example, a metabolic product, a hormone, an antigen, an antibody, and the like. Alternatively, the analyte can be introduced into the body or exogenous, for example, a contrast agent for imaging, a radioisotope, a chemical agent, a fluorocarbon-based synthetic blood, or a drug or pharmaceutical composition, including but not limited to: insulin; ethanol; cannabis (marijuana, tetrahydrocannabinol, hashish); inhalants (nitrous oxide, amyl nitrite, butyl nitrite, chlorohydrocarbons, hydrocarbons); cocaine (crack cocaine); stimulants (amphetamines, methamphetamines, Ritalin, Cylert, Preludin, Didrex, PreState, Voranil, Sandrex, Plegine); depressants (barbituates, methaqualone, tranquilizers such as Valium, Librium, Miltown, Serax, Equanil, Tranxene); hallucinogens (phencyclidine, lysergic acid, mescaline, peyote, psilocybin); narcotics (heroin, codeine, morphine, opium, meperidine, Percocet, Percodan, Tussionex, Fentanyl, Darvon, Talwin, Lomotil); designer drugs (analogs of fentanyl, meperidine, amphetamines, methamphetamines, and phencyclidine, for example, Ecstasy); anabolic steroids; and nicotine. The metabolic products of drugs and pharmaceutical compositions are also contemplated analytes. Analytes such as neurochemicals and other chemicals generated within the body can also be analyzed, such as, for example, ascorbic acid, uric acid, dopamine, noradrenaline, 3-methoxytyramine (3MT), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), 5-hydroxytryptamine (5HT), and 5-hydroxyindoleacetic acid (FHIAA).

The phrase ‘continuous (or continual) analyte sensing’ as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and is not to be limited to a special or customized meaning), and refers without limitation to the period in which monitoring of analyte concentration is continuously, continually, and or intermittently (but regularly) performed, for example, about every 5 to 10 minutes.

The terms ‘operable connection,’ ‘operably connected,’ and ‘operably linked’ as used herein are broad terms, and are to be given their ordinary and customary meaning to a person of ordinary skill in the art (and are not to be limited to a special or customized meaning), and refer without limitation to one or more components linked to another component(s) in a manner that allows transmission of signals between the components. For example, one or more electrodes can be used to detect the amount of analyte in a sample and convert that information into a signal; the signal can then be transmitted to a circuit. In this case, the electrode is ‘operably linked’ to the electronic circuitry.

The term ‘host’ as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and is not to be limited to a special or customized meaning), and refers without limitation to animals (e.g., humans) and plants.

The terms ‘electrochemically reactive surface’ and electroactive surface' as used herein are broad terms, and are to be given their ordinary and customary meaning to a person of ordinary skill in the art (and are not to be limited to a special or customized meaning), and refer without limitation to the surface of an electrode where an electrochemical reaction takes place. As one example, in a working electrode, H₂O₂ (hydrogen peroxide) produced by an enzyme-catalyzed reaction of an analyte being detected reacts and thereby creates a measurable electric current. For example, in the detection of glucose, glucose oxidase produces H₂O₂ as a byproduct. The H₂O₂ reacts with the surface of the working electrode to produce two protons (2H⁺), two electrons (2e⁻), and one molecule of oxygen (O₂), which produces the electric current being detected. In the case of the counter electrode, a reducible species, for example, O₂ is reduced at the electrode surface in order to balance the current being generated by the working electrode.

The terms ‘sensing region,’ ‘sensor’, and ‘sensing mechanism’ as used herein are broad terms, and are to be given their ordinary and customary meaning to a person of ordinary skill in the art (and are not to be limited to a special or customized meaning), and refer without limitation to the region or mechanism of a monitoring device responsible for the detection of a particular analyte.

The terms ‘raw data stream’ and ‘data stream’ as used herein are broad terms, and are to be given their ordinary and customary meaning to a person of ordinary skill in the art (and are not to be limited to a special or customized meaning), and refer without limitation to an analog or digital signal directly related to the measured glucose concentration from the glucose sensor. In one example, the raw data stream is digital data in ‘counts’ converted by an A/D converter from an analog signal (for example, voltage or amps) representative of a glucose concentration. The terms broadly encompass a plurality of time spaced data points from a substantially continuous glucose sensor, which comprises individual measurements taken at time intervals ranging from fractions of a second up to, for example, 1, 2, or 5 minutes or longer.

The term ‘counts’ as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and is not to be limited to a special or customized meaning), and refers without limitation to a unit of measurement of a digital signal. In one example, a raw data stream measured in counts is directly related to a voltage (for example, converted by an A/D converter), which is directly related to current from the working electrode. In another example, counter electrode voltage measured in counts is directly related to a voltage.

The term ‘electrical potential’ as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and is not to be limited to a special or customized meaning), and refers without limitation to the electrical potential difference between two points in a circuit which is the cause of the flow of a current.

The phrase ‘distal to’ as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and is not to be limited to a special or customized meaning), and refers without limitation to the spatial relationship between various elements in comparison to a particular point of reference. For example, some embodiments of a sensor include a membrane system having a bioprotective domain and an enzyme domain. If the sensor is deemed to be the point of reference and the bioprotective domain is positioned farther from the sensor than the enzyme domain, then the bioprotective domain is more distal to the sensor than the enzyme domain.

The phrase ‘proximal to’ as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and is not to be limited to a special or customized meaning), and refers without limitation to the spatial relationship between various elements in comparison to a particular point of reference. For example, some embodiments of a device include a membrane system having a bioprotective domain and an enzyme domain. If the sensor is deemed to be the point of reference and the enzyme domain is positioned nearer to the sensor than the bioprotective domain, then the enzyme domain is more proximal to the sensor than the bioprotective domain.

The terms ‘interferents’ and ‘interfering species’ as used herein are broad terms, and are to be given their ordinary and customary meaning to a person of ordinary skill in the art (and are not to be limited to a special or customized meaning), and refer without limitation to effects or species that interfere with the measurement of an analyte of interest in a sensor to produce a signal that does not accurately represent the analyte measurement. In an exemplary electrochemical sensor, interfering species can include compounds with an oxidation potential that overlaps with that of the analyte to be measured.

The term ‘domain’ as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and is not to be limited to a special or customized meaning), and refers without limitation to regions of a membrane that can be layers, uniform or non-uniform gradients (i.e., anisotropic) or provided as portions of the membrane.

The terms ‘sensing membrane’ and ‘membrane system’ as used herein are broad terms, and are to be given their ordinary and customary meaning to a person of ordinary skill in the art (and are not to be limited to a special or customized meaning), and refers without limitation to a permeable or semi-permeable membrane that can comprise one or more domains and constructed of materials of a few microns thickness or more, which are permeable to oxygen and may or may not be permeable to an analyte of interest. In one example, the sensing membrane or membrane system may comprise an immobilized glucose oxidase enzyme, which enables an electrochemical reaction to occur to measure a concentration of glucose.

The term ‘baseline’ as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and is not to be limited to a special or customized meaning), and refers without limitation to the component of an analyte sensor signal that is not related to the analyte concentration. In one example of a glucose sensor, the baseline is composed substantially of signal contribution due to factors other than glucose (for example, interfering species, non-reaction-related hydrogen peroxide, or other electroactive species with an oxidation potential that overlaps with hydrogen peroxide). In some embodiments wherein a calibration is defined by solving for the equation y=mx+b, the value of b represents the baseline of the signal.

The term ‘sensitivity’ as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and is not to be limited to a special or customized meaning), and refers without limitation to an amount of electrical current produced by a predetermined amount (unit) of the measured analyte. For example, in one embodiment, a sensor has a sensitivity (or slope) of from about 1 to about 100 picoAmps of current for every 1 mg/dL of glucose analyte.

As employed herein, the following abbreviations apply: Eq and Eqs (equivalents); mEq (milliequivalents); M (molar); mM (millimolar) μM (micromolar); N (Normal); mol (moles); mmol (millimoles); μmol (micromoles); nmol (nanomoles); g (grams); mg (milligrams); μg (micrograms); Kg (kilograms); L (liters); mL (milliliters); dL (deciliters); μL (microliters); cm (centimeters); mm (millimeters); μm (micrometers); nm (nanometers); h and hr (hours); min. (minutes); s and sec. (seconds); ° C. (degrees Centigrade).

Overview

Membrane systems of the various embodiments are suitable for use with implantable devices in contact with a biological fluid. For example, the membrane systems can be utilized with implantable devices, such as devices for monitoring and determining analyte levels in a biological fluid, for example, devices for monitoring glucose levels for individuals having diabetes. In some embodiments, the analyte-measuring device is a continuous device. The analyte-measuring device can employ any suitable sensing element to provide the raw signal, including but not limited to those involving enzymatic, chemical, physical, electrochemical, spectrophotometric, polarimetric, calorimetric, radiometric, immunochemical, or like elements.

Although some of the description that follows is directed at glucose-measuring devices, including the described membrane systems and methods for their use, these membrane systems are not limited to use in devices that measure or monitor glucose. These membrane systems are suitable for use in any of a variety of devices, including, for example, devices that detect and quantify other analytes present in biological fluids (e.g., cholesterol, amino acids, alcohol, galactose, and lactate), cell transplantation devices (see, for example, U.S. Pat. Nos. 6,015,572, 5,964,745, and 6,083,523), drug delivery devices (see, for example, U.S. Pat. Nos. 5,458,631, 5,820,589, and 5,972,369), and the like.

In one embodiment, the analyte sensor is an implantable glucose sensor, such as described with reference to U.S. Pat. No. 6,001,067 and U.S. Patent Publication No. US-2005-0027463-A1. In another embodiment, the analyte sensor is a glucose sensor, such as described with reference to U.S. Patent Publication No. US-2006-0020187-A1. In still other embodiments, the sensor is configured to be implanted in a host vessel or extra-corporeally, such as is described in U.S. Patent Publication No. US-2007-0027385-A1, U.S. Patent Publication No. US-2008-0119703-A1, U.S. Patent Publication No. US-2008-0108942-A1, and U.S. Patent Publication No. US-2007-0197890-A1. In some embodiments, the sensor is configured as a dual-electrode sensor, such as described in U.S. Patent Publication No. US-2005-0143635-A1, U.S. Patent Publication No. US-2007-0027385-A1, U.S. Patent Publication No. US-2007-0213611-A1, and U.S. Patent Publication No. US-2008-0083617-A1. In one alternative embodiment, the continuous glucose sensor comprises a sensor such as described in U.S. Pat. No. 6,565,509 to Say et al., for example. In another alternative embodiment, the continuous glucose sensor comprises a subcutaneous sensor such as described with reference to U.S. Pat. No. 6,579,690 to Bonnecaze et al. or U.S. Pat. No. 6,484,046 to Say et al., for example. In another alternative embodiment, the continuous glucose sensor comprises a refillable subcutaneous sensor such as described with reference to U.S. Pat. No. 6,512,939 to Colvin et al., for example. In yet another alternative embodiment, the continuous glucose sensor comprises an intravascular sensor such as described with reference to U.S. Pat. No. 6,477,395 to Schulman et al., for example. In another alternative embodiment, the continuous glucose sensor comprises an intravascular sensor such as described with reference to U.S. Pat. No. 6,424,847 to Mastrototaro et al. In some embodiments, the electrode system can be used with any of a variety of known in vivo analyte sensors or monitors, such as U.S. Pat. No. 7,157,528 to Ward; U.S. Pat. No. 6,212,416 to Ward et al.; U.S. Pat. No. 6,119,028 to Schulman et al.; U.S. Pat. No. 6,400,974 to Lesho; U.S. Pat. No. 6,595,919 to Berner et al.; U.S. Pat. No. 6,141,573 to Kurnik et al.; U.S. Pat. No. 6,122,536 to Sun et al.; European Patent Application EP 1153571 to Varall et al.; U.S. Pat. No. 6,512,939 to Colvin et al.; U.S. Pat. No. 5,605,152 to Slate et al.; U.S. Pat. No. 4,431,004 to Bessman et al.; U.S. Pat. No. 4,703,756 to Gough et al.; U.S. Pat. No. 6,514,718 to Heller et al.; U.S. Pat. No. 5,985,129 to Gough et al.; WO Patent Application Publication No. 04/021877 to Caduff; U.S. Pat. No. 5,494,562 to Maley et al.; U.S. Pat. No. 6,120,676 to Heller et al.; and U.S. Pat. No. 6,542,765 to Guy et al. In general, it is understood that the disclosed embodiments are applicable to a variety of continuous analyte measuring device configurations.

In some embodiments, a long term sensor (e.g., wholly implantable or intravascular) is configured and arranged to function for a time period of from about 30 days or less to about one year or more (e.g., a sensor session). In some embodiments, a short term sensor (e.g., one that is transcutaneous or intravascular) is configured and arranged to function for a time period of from about a few hours to about 30 days, including a time period of about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28 or 29 days (e.g., a sensor session). As used herein, the term ‘sensor session’ is a broad term and refers without limitation to the period of time the sensor is applied to (e.g., implanted in) the host or is being used to obtain sensor values. For example, in some embodiments, a sensor session extends from the time of sensor implantation (e.g., including insertion of the sensor into subcutaneous tissue and placing the sensor into fluid communication with a host's circulatory system) to the time when the sensor is removed.

Exemplary Glucose Sensor Configuration

FIG. 1 is an expanded view of an exemplary embodiment of a continuous analyte sensor 34, also referred to as an analyte sensor, illustrating the sensing mechanism. In some embodiments, the sensing mechanism is adapted for insertion under the host's skin, and the remaining body of the sensor (e.g., electronics, etc.) can reside ex vivo. In the illustrated embodiment, the analyte sensor 34 includes two electrodes, i.e., a working electrode 38 and at least one additional electrode 30, which may function as a counter or reference electrode, hereinafter referred to as the reference electrode 30.

It is contemplated that the electrode may be formed to have any of a variety of cross-sectional shapes. For example, in some embodiments, the electrode may be formed to have a circular or substantially circular shape, but in other embodiments, the electrode may be formed to have a cross-sectional shape that resembles an ellipse, a polygon (e.g., triangle, square, rectangle, parallelogram, trapezoid, pentagon, hexagon, octagon), or the like. In various embodiments, the cross-sectional shape of the electrode may be symmetrical, but in other embodiments, the cross-sectional shape may be asymmetrical. In some embodiments, each electrode may be formed from a fine wire with a diameter of from about 0.001 or less to about 0.050 inches or more, for example, and is formed from, e.g., a plated insulator, a plated wire, or bulk electrically conductive material. In some embodiments, the wire used to form a working electrode may be about 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.01, 0.015, 0.02, 0.025, 0.03, 0.035, 0.04, or 0.045 inches in diameter. In some embodiments, the working electrode may comprise a wire formed from a conductive material, such as platinum, platinum-black, platinum-iridium, palladium, graphite, gold, carbon, conductive polymer, alloys, or the like. Although the illustrated electrode configuration and associated text describe one method of forming a sensor, any of a variety of known sensor configurations can be employed with the analyte sensor system.

The working electrode 38 is configured to measure the concentration of an analyte, such as, but not limited to glucose, uric acid, cholesterol, lactate, and the like. In an enzymatic electrochemical sensor for detecting glucose, for example, the working electrode may measure the hydrogen peroxide produced by an enzyme catalyzed reaction of the analyte being detected and creates a measurable electric current. For example, in the detection of glucose wherein glucose oxidase (GOX) produces H₂O₂ as a byproduct, the H₂O₂ reacts with the surface of the working electrode producing two protons (2H⁺), two electrons (2e⁻) and one molecule of oxygen (O₂), which produces the electric current being detected.

An insulator may be provided to electrically insulate the working and reference electrodes. In this exemplary embodiment, the working electrode 38 is covered with an insulating material, for example, a non-conductive polymer. Dip-coating, spray-coating, vapor-deposition, or other coating or deposition techniques can be used to deposit the insulating material on the working electrode. In one embodiment, the insulating material comprises parylene, which can be an advantageous polymer coating because of its strength, lubricity, and electrical insulation properties. Generally, parylene is produced by vapor deposition and polymerization of para-xylylene (or its substituted derivatives). However, any suitable insulating material can be used, for example, fluorinated polymers, polyethyleneterephthalate, polyurethane, polyimide, other nonconducting polymers, or the like. Glass or ceramic materials can also be employed. Other materials suitable for use include surface energy modified coating systems such as those marketed under the trade names AMC18, AMC148, AMC141, and AMC321 by Advanced Materials Components Express of Bellafonte, PA. In some alternative embodiments, however, the working electrode may not require a coating of insulator.

In some embodiments, the reference electrode 30, which may function as a reference electrode alone, or as a dual reference and counter electrode, is formed from silver, silver/silver chloride, or the like. In some embodiments, the electrodes are juxtapositioned or twisted with or around each other, but it is contemplated, however, that other configurations are also possible. In one embodiment, the reference electrode 30 is helically wound around the working electrode 38. The assembly of wires may then be optionally coated together with an insulating material, similar to that described above, in order to provide an insulating attachment (e.g., securing together of the working and reference electrodes).

In embodiments wherein an outer insulator is disposed, a portion of the coated assembly structure can be stripped or otherwise removed, for example, by hand, excimer lasing, chemical etching, laser ablation, grit-blasting, or the like, to expose the electroactive surfaces. Alternatively, a portion of the electrode can be masked prior to depositing the insulator in order to maintain an exposed electroactive surface area.

In some embodiments, a radial window is formed through the insulating material to expose a circumferential electroactive surface of the working electrode. Additionally, sections of electroactive surface of the reference electrode are exposed. For example, the sections of electroactive surface can be masked during deposition of an outer insulating layer or etched after deposition of an outer insulating layer. In some applications, cellular attack or migration of cells to the sensor can cause reduced sensitivity or function of the device, particularly after the first day of implantation. However, when the exposed electroactive surface is distributed circumferentially about the sensor (e.g., as in a radial window), the available surface area for reaction can be sufficiently distributed so as to minimize the effect of local cellular invasion of the sensor on the sensor signal. Alternatively, a tangential exposed electroactive window can be formed, for example, by stripping only one side of the coated assembly structure. In other alternative embodiments, the window can be provided at the tip of the coated assembly structure such that the electroactive surfaces are exposed at the tip of the sensor. Other methods and configurations for exposing electroactive surfaces can also be employed. Additional embodiments of electrode configurations are described U.S. Patent Publication No. US-2011-0027127-A1, which is incorporated herein by reference in its entirety.

In some alternative embodiments, additional electrodes can be included within the assembly, for example, a three-electrode system (working, reference, and counter electrodes) and an additional working electrode (e.g., an electrode which can be used to generate oxygen, which is configured as a baseline subtracting electrode, or which is configured for measuring additional analytes). U.S. Pat. No. 7,081,195, U.S. Patent Publication No. US-2005-0143635-A1 and U.S. Patent Publication No. US-2007-0027385-A1 describe some systems and methods for implementing and using additional working, counter, and reference electrodes. In one implementation wherein the sensor comprises two working electrodes, the two working electrodes are juxtapositioned, around which the reference electrode is disposed (e.g., helically wound). In some embodiments wherein two or more working electrodes are provided, the working electrodes can be formed in a double-, triple-, quad-, etc. helix configuration along the length of the sensor (for example, surrounding a reference electrode, insulated rod, or other support structure). The resulting electrode system can be configured with an appropriate membrane system, wherein the first working electrode is configured to measure a first signal comprising glucose and baseline signals, and the additional working electrode is configured to measure a baseline signal consisting of the baseline signal only. In these embodiments, the second working electrode may be configured to be substantially similar to the first working electrode, but without an enzyme disposed thereon. In this way, the baseline signal can be determined and subtracted from the first signal to generate a difference signal, i.e., a glucose-only signal that is substantially not subject to fluctuations in the baseline or interfering species on the signal, such as described in U.S. Patent Publication No. US-2005-0143635-A1, U.S. Patent Publication No. US-2007-0027385-A1, and U.S. Patent Publication No. US-2007-0213611-A1, and U.S. Patent Publication No. US-2008-0083617-A1.

It has been found that in some electrode systems involving two working electrodes, i.e., in some dual-electrode systems, the working electrodes may sometimes be slightly different from each other. For instance, two working electrodes, even when manufactured from a single facility may slightly differ in thickness or permeability because of the electrodes' high sensitivity to environmental conditions (e.g., temperature, humidity) during fabrication. Accordingly, the working electrodes of a dual-electrode system may sometimes have varying diffusion, membrane thickness, and diffusion characteristics. As a result, the above-described difference signal (i.e., a glucose-only signal, generated from subtracting the baseline signal from the first signal) may not be completely accurate. To mitigate this, it is contemplated that in some dual-electrode systems, both working electrodes may be fabricated with one or more membranes that each includes a bioprotective layer, which is described in more detail elsewhere herein.

It is contemplated that the sensing region may include any of a variety of electrode configurations. For example, in some embodiments, in addition to one or more glucose-measuring working electrodes, the sensing region may also include a reference electrode or other electrodes associated with the working electrode. In these particular embodiments, the sensing region may also include a separate reference or counter electrode associated with one or more optional auxiliary working electrodes. In other embodiments, the sensing region may include a glucose-measuring working electrode, an auxiliary working electrode, two counter electrodes (one for each working electrode), and one shared reference electrode. In yet other embodiments, the sensing region may include a glucose-measuring working electrode, an auxiliary working electrode, two reference electrodes, and one shared counter electrode.

U.S. Patent Publication No. US-2008-0119703-A1 and U.S. Patent Publication No. US-2005-0245799-A1 describes additional configurations for using the continuous sensor in different body locations. In some embodiments, the sensor is configured for transcutaneous implantation in the host. In alternative embodiments, the sensor is configured for insertion into the circulatory system, such as a peripheral vein or artery. However, in other embodiments, the sensor is configured for insertion into the central circulatory system, such as but not limited to the vena cava. In still other embodiments, the sensor can be placed in an extracorporeal circulation system, such as but not limited to an intravascular access device providing extracorporeal access to a blood vessel, an intravenous fluid infusion system, an extracorporeal blood chemistry analysis device, a dialysis machine, a heart-lung machine (i.e., a device used to provide blood circulation and oxygenation while the heart is stopped during heart surgery), etc. In still other embodiments, the sensor can be configured to be wholly implantable, as described in U.S. Pat. No. 6,001,067.

FIG. 2A is a cross-sectional view through the sensor of FIG. 1 on line 2-2, illustrating one embodiment of the membrane system 32. In this particular embodiment, the membrane system includes an enzyme domain 42, a diffusion resistance domain 44, and a bioprotective domain 46 located around the working electrode 38, all of which are described in more detail elsewhere herein. In some embodiments, a unitary diffusion resistance domain and bioprotective domain may be included in the membrane system (e.g., wherein the functionality of both domains is incorporated into one domain, i.e., the bioprotective domain). In some embodiments, the sensor is configured for short-term implantation (e.g., from about 1 to 30 days). However, it is understood that the membrane system 32 can be modified for use in other devices, for example, by including only one or more of the domains, or additional domains.

In some embodiments, the membrane system may include a bioprotective domain 46, also referred to as a cell-impermeable domain or biointerface domain, comprising a surface-modified base polymer as described in more detail elsewhere herein. However, the sensing membranes 32 of some embodiments can also include a plurality of domains or layers including, for example, an electrode domain (e.g., as illustrated in the FIG. 2C), an interference domain (e.g., as illustrated in FIG. 2B), or a cell disruptive domain (not shown), such as described in more detail elsewhere herein and in U.S. Patent Publication No. US-2006-0036145-A1.

It is to be understood that sensing membranes modified for other sensors, for example, may include fewer or additional layers. For example, in some embodiments, the membrane system may comprise one electrode layer, one enzyme layer, and two bioprotective layers, but in other embodiments, the membrane system may comprise one electrode layer, two enzyme layers, and one bioprotective layer. In some embodiments, the bioprotective layer may be configured to function as the diffusion resistance domain and control the flux of the analyte (e.g., glucose) to the underlying membrane layers.

In some embodiments, one or more domains of the sensing membranes may be formed from materials such as silicone, polytetrafluoroethylene, polyethylene-co-tetrafluoroethylene, polyolefin, polyester, polycarbonate, biostable polytetrafluoroethylene, homopolymers, copolymers, terpolymers of polyurethanes, polypropylene (PP), polyvinylchloride (PVC), polyvinylidene fluoride (PVDF), polybutylene terephthalate (PBT), polymethylmethacrylate (PMMA), polyether ether ketone (PEEK), polyurethanes, cellulosic polymers, poly(ethylene oxide), poly(propylene oxide) and copolymers and blends thereof, polysulfones and block copolymers thereof including, for example, di-block, tri-block, alternating, random and graft copolymers.

In some embodiments, the sensing membrane can be deposited on the electroactive surfaces of the electrode material using known thin or thick film techniques (for example, spraying, electro-depositing, dipping, or the like). It should be appreciated that the sensing membrane located over the working electrode does not have to have the same structure as the sensing membrane located over the reference electrode; for example, the enzyme domain deposited over the working electrode does not necessarily need to be deposited over the reference or counter electrodes.

Although the exemplary embodiments illustrated in FIGS. 2A-2C involve circumferentially extending membrane systems, the membranes described herein may be applied to any planar or non-planar surface, for example, the substrate-based sensor structure of U.S. Pat. No. 6,565,509 to Say et al.

Sensor Electronics

In general, analyte sensor systems have electronics associated therewith, also referred to as a ‘computer system’ that can include hardware, firmware, or software that enable measurement and processing of data associated with analyte levels in the host. In one exemplary embodiment of an electrochemical sensor, the electronics include a potentiostat, a power source for providing power to the sensor, and other components useful for signal processing. In additional embodiments, some or all of the electronics can be in wired or wireless communication with the sensor or other portions of the electronics. For example, a potentiostat disposed on the device can be wired to the remaining electronics (e.g., a processor, a recorder, a transmitter, a receiver, etc.), which reside on the bedside. In another example, some portion of the electronics is wirelessly connected to another portion of the electronics (e.g., a receiver), such as by infrared (IR) or RF. It is contemplated that other embodiments of electronics may be useful for providing sensor data output, such as those described in U.S. Patent Publication No. US-2005-0192557-A1, U.S. Patent Publication No. US-2005-0245795-A1; U.S. Patent Publication No. US-2005-0245795-A1, and U.S. Patent Publication No. US-2005-0245795-A1, U.S. Patent Publication No. US-2008-0119703-A1, and U.S. Patent Publication No. US-2008-0108942-A1.

In one embodiment, a potentiostat is operably connected to the electrode(s) (such as described elsewhere herein), which biases the sensor to enable measurement of a current signal indicative of the analyte concentration in the host (also referred to as the analog portion). In some embodiments, the potentiostat includes a resistor that translates the current into voltage. In some alternative embodiments, a current to frequency converter is provided that is configured to continuously integrate the measured current, for example, using a charge counting device. In some embodiments, the electronics include an A/D converter that digitizes the analog signal into a digital signal, also referred to as ‘counts’ for processing. Accordingly, the resulting raw data stream in counts, also referred to as raw sensor data, is directly related to the current measured by the potentiostat.

In general, the electronics include a processor module that includes the central control unit that controls the processing of the sensor system. In some embodiments, the processor module includes a microprocessor, however a computer system other than a microprocessor can be used to process data as described herein, for example an ASIC can be used for some or all of the sensor's central processing. The processor typically provides semi-permanent storage of data, for example, storing data such as sensor identifier (ID) and programming to process data streams (for example, programming for data smoothing or replacement of signal artifacts such as is described in U.S. Patent Publication No. US-2005-0043598-A1). The processor additionally can be used for the system's cache memory, for example for temporarily storing recent sensor data. In some embodiments, the processor module comprises memory storage components such as ROM, RAM, dynamic-RAM, static-RAM, non-static RAM, EEPROM, rewritable ROMs, flash memory, and the like.

In some embodiments, the processor module comprises a digital filter, for example, an infinite impulse response (IIR) or finite impulse response (FIR) filter, configured to smooth the raw data stream. Generally, digital filters are programmed to filter data sampled at a predetermined time interval (also referred to as a sample rate). In some embodiments, wherein the potentiostat is configured to measure the analyte at discrete time intervals, these time intervals determine the sample rate of the digital filter. In some alternative embodiments, wherein the potentiostat is configured to continuously measure the analyte, for example, using a current-to-frequency converter as described above, the processor module can be programmed to request a digital value from the A/D converter at a predetermined time interval, also referred to as the acquisition time. In these alternative embodiments, the values obtained by the processor are advantageously averaged over the acquisition time due the continuity of the current measurement. Accordingly, the acquisition time determines the sample rate of the digital filter.

In some embodiments, the processor module is configured to build the data packet for transmission to an outside source, for example, an RF transmission to a receiver. Generally, the data packet comprises a plurality of bits that can include a preamble, a unique identifier identifying the electronics unit, the receiver, or both, (e.g., sensor ID code), data (e.g., raw data, filtered data, or an integrated value) or error detection or correction. The data (transmission) packet can have a length of from about 8 bits to about 128 bits, or, for example, of about 48 bits; however, larger or smaller packets can be desirable in certain embodiments. The processor module can be configured to transmit any combination of raw or filtered data. In one exemplary embodiment, the transmission packet contains a fixed preamble, a unique ID of the electronics unit, a single five-minute average (e.g., integrated) sensor data value, and a cyclic redundancy code (CRC).

In some embodiments, the processor further performs the processing, such as storing data, analyzing data streams, calibrating analyte sensor data, estimating analyte values, comparing estimated analyte values with time corresponding measured analyte values, analyzing a variation of estimated analyte values, downloading data, and controlling the user interface by providing analyte values, prompts, messages, warnings, alarms, and the like. In such cases, the processor includes hardware and software that performs the processing described herein, for example flash memory provides permanent or semi-permanent storage of data, storing data such as sensor ID, receiver ID, and programming to process data streams (for example, programming for performing estimation and other algorithms described elsewhere herein) and random access memory (RAM) stores the system's cache memory and is helpful in data processing. Alternatively, some portion of the data processing (such as described with reference to the processor elsewhere herein) can be accomplished at another (e.g., remote) processor and can be configured to be in wired or wireless connection therewith.

In some embodiments, an output module, which is integral with or operatively connected with the processor, includes programming for generating output based on the data stream received from the sensor system and it's processing incurred in the processor. In some embodiments, output is generated via a user interface.

Noise

Generally, implantable sensors measure a signal related to an analyte of interest in a host. For example, an electrochemical sensor can measure glucose, creatinine, or urea in a host, such as an animal (e.g., a human). Generally, the signal is converted mathematically to a numeric value indicative of analyte status, such as analyte concentration, as described in more detail elsewhere herein. In general, the signal generated by conventional analyte sensors contains some noise. Noise is clinically important because it can induce error and can reduce sensor performance, such as by providing a signal that causes the analyte concentration to appear higher or lower than the actual analyte concentration. For example, upward or high noise (e.g., noise that causes the signal to increase) can cause the reading of the host's glucose concentration to appear higher than the actual value, which in turn can lead to improper treatment decisions. Similarly, downward or low noise (e.g., noise that causes the signal to decrease) can cause the reading of the host's glucose concentration to appear lower than its actual value, which in turn can also lead to improper treatment decisions. Accordingly, noise reduction is desirable.

In general, the signal detected by the sensor can be broken down into its component parts. For example, in one embodiment of an enzymatic electrochemical analyte sensor, after sensor break-in is complete, the total signal can be divided into an ‘analyte component,’ which is representative of analyte (e.g., glucose) concentration, and a ‘noise component,’ which is caused by non-analyte-related species that have a redox potential that substantially overlaps with the redox potential of the analyte (or measured species, e.g., H₂O₂) at an applied voltage. The noise component can be further divided into its component parts, e.g., constant and non-constant noise. It is not unusual for a sensor to experience a certain level of noise. In general, ‘constant noise’ (sometimes referred to as constant background or baseline) is caused by non-analyte-related factors that are relatively stable over time, including but not limited to electroactive species that arise from generally constant (e.g., daily) metabolic processes. Constant noise can vary widely between hosts. In contrast, ‘non-constant noise’ (sometimes referred to as non-constant background) is generally caused by non-constant, non-analyte-related species (e.g., non-constant noise-causing electroactive species) that may arise during transient events, such as during host metabolic processes (e.g., wound healing or in response to an illness), or due to ingestion of certain compounds (e.g., certain drugs). In some circumstances, noise can be caused by a variety of noise-causing electroactive species, which are discussed in detail elsewhere herein.

FIG. 3 is a graph illustrating the components of a signal measured by a transcutaneous glucose sensor (after sensor break-in was complete), in a non-diabetic volunteer host. The Y-axis indicates the signal amplitude (in counts) detected by the sensor. The total signal collected by the sensor is represented by line 1000, which includes components related to glucose, constant noise, and non-constant noise, which are described in more detail elsewhere herein. In some embodiments, the total signal is a raw data stream, which can include an averaged or integrated signal, for example, using a charge-counting device.

The non-constant noise component of the total signal is represented by line 1010. The non-constant noise component 1010 of the total signal 1000 can be obtained by filtering the total signal 1000 to obtain a filtered signal 1020 using any of a variety of known filtering techniques, and then subtracting the filtered signal 1020 from the total signal 1000. In some embodiments, the total signal can be filtered using linear regression analysis of the n (e.g., 10) most recent sampled sensor values. In some embodiments, the total signal can be filtered using non-linear regression. In some embodiments, the total signal can be filtered using a trimmed regression, which is a linear regression of a trimmed mean (e.g., after rejecting wide excursions of any point from the regression line). In this embodiment, after the sensor records glucose measurements at a predetermined sampling rate (e.g., every 30 seconds), the sensor calculates a trimmed mean (e.g., removes highest and lowest measurements from a data set) and then regresses the remaining measurements to estimate the glucose value. In some embodiments, the total signal can be filtered using a non-recursive filter, such as a finite impulse response (FIR) filter. An FIR filter is a digital signal filter, in which every sample of output is the weighted sum of past and current samples of input, using only some finite number of past samples. In some embodiments, the total signal can be filtered using a recursive filter, such as an infinite impulse response (IIR) filter. An IIR filter is a type of digital signal filter, in which every sample of output is the weighted sum of past and current samples of input. In some embodiments, the total signal can be filtered using a maximum-average (max-average) filtering algorithm, which smoothes data based on the discovery that the substantial majority of signal artifacts observed after implantation of glucose sensors in humans, for example, is not distributed evenly above and below the actual blood glucose levels. It has been observed that many data sets are actually characterized by extended periods in which the noise appears to trend downwardly from maximum values with occasional high spikes. To overcome these downward trending signal artifacts, the max-average calculation tracks with the highest sensor values, and discards the bulk of the lower values. Additionally, the max-average method is designed to reduce the contamination of the data with unphysiologically high data from the high spikes. The max-average calculation smoothes data at a sampling interval (e.g., every 30 seconds) for transmission to the receiver at a less frequent transmission interval (e.g., every 5 minutes), to minimize the effects of low non-physiological data. First, the microprocessor finds and stores a maximum sensor counts value in a first set of sampled data points (e.g., 5 consecutive, accepted, thirty-second data points). A frame shift time window finds a maximum sensor counts value for each set of sampled data (e.g., each 5-point cycle length) and stores each maximum value. The microprocessor then computes a rolling average (e.g., 5-point average) of these maxima for each sampling interval (e.g., every 30 seconds) and stores these data. Periodically (e.g., every 10^(th) interval), the sensor outputs to the receiver the current maximum of the rolling average (e.g., over the last 10 thirty-second intervals as a smoothed value for that time period (e.g., 5 minutes)). In some embodiments, the total signal can be filtered using a ‘Cone of Possibility Replacement Method,’ which utilizes physiological information along with glucose signal values in order define a ‘cone’ of physiologically feasible glucose signal values within a human. Particularly, physiological information depends upon the physiological parameters obtained from continuous studies in the literature as well as our own observations. A first physiological parameter uses a maximal sustained rate of change of glucose in humans (e.g., about 4 to 6 mg/dl/min) and a maximum sustained acceleration of that rate of change (e.g., about 0.1 to 0.2 mg/min/min). A second physiological parameter uses the knowledge that rate of change of glucose is lowest at the maxima and minima, which are the areas of greatest risk in patient treatment. A third physiological parameter uses the fact that the best solution for the shape of the curve at any point along the curve over a certain time period (e.g., about 20-25 minutes) is a straight line. It is noted that the maximum rate of change can be narrowed in some instances. Therefore, additional physiological data can be used to modify the limits imposed upon the Cone of Possibility Replacement Method for sensor glucose values. For example, the maximum per minute rate of change can be lower when the subject is lying down or sleeping; on the other hand, the maximum per minute rate change can be higher when the subject is exercising, for example. In some embodiments, the total signal can be filtered using reference changes in electrode potential to estimate glucose sensor data during positive detection of signal artifacts from an electrochemical glucose sensor, the method hereinafter referred to as reference drift replacement; in this embodiment, the electrochemical glucose sensor comprises working, counter, and reference electrodes. This method exploits the function of the reference electrode as it drifts to compensate for counter electrode limitations during oxygen deficits, pH changes, or temperature changes. In alternative implementations of the reference drift method, a variety of algorithms can therefore be implemented based on the changes measured in the reference electrode. Linear algorithms, and the like, are suitable for interpreting the direct relationship between reference electrode drift and the non-glucose rate limiting signal noise such that appropriate conversion to signal noise compensation can be derived. Additional description of signal filtering can be found in U.S. Patent Publication No. US-2005-0043598-A1.

The constant noise signal component 1030 can be obtained by calibrating the sensor signal using reference data, such as one or more blood glucose values obtained from a hand-held blood glucose meter, or the like, from which the baseline ‘b’ of a regression can be obtained, representing the constant noise signal component 1030.

The analyte signal component 1040 can be obtained by subtracting the constant noise signal component 1030 from the filtered signal 1020.

In general, non-constant noise is caused by interfering species (non-constant noise-causing species), which can be compounds, such as drugs that have been administered to the host, or intermittently produced products of various host metabolic processes. Exemplary interferents include but are not limited to a variety of drugs (e.g., acetaminophen), H₂O₂ from exterior sources (e.g., produced outside the sensor membrane system), and reactive metabolic species (e.g., reactive oxygen and nitrogen species, some hormones, etc.). Some known interfering species for a glucose sensor include but are not limited to acetaminophen, ascorbic acid, bilirubin, cholesterol, creatinine, dopamine, ephedrine, ibuprofen, L-dopa, methyldopa, salicylate, tetracycline, tolazamide, tolbutamide, triglycerides, and uric acid.

In some experiments of implantable glucose sensors, it was observed that noise increased when some hosts were intermittently sedentary, such as during sleep or sitting for extended periods. When the host began moving again, the noise quickly dissipated. Noise that occurs during intermittent, sedentary periods (sometimes referred to as intermittent sedentary noise) can occur during relatively inactive periods, such as sleeping. Non-constant, non-analyte-related factors can cause intermittent sedentary noise, such as was observed in one exemplary study of non-diabetic individuals implanted with enzymatic-type glucose sensors built without enzyme. These sensors (without enzyme) could not react with or measure glucose and therefore provided a signal due to non-glucose effects only (e.g., constant and non-constant noise). During sedentary periods (e.g., during sleep), extensive, sustained signal was observed on the sensors. Then, when the host got up and moved around, the signal rapidly corrected. As a control, in vitro experiments were conducted to determine if a sensor component might have leached into the area surrounding the sensor and caused the noise, but none was detected. From these results, it is believed that a host-produced non-analyte related reactant was diffusing to the electrodes and producing the unexpected non-constant noise signal.

Interferents

Interferents are molecules or other species that may cause a sensor to generate a false positive or negative analyte signal (e.g., a non-analyte-related signal). Some interferents are known to become reduced or oxidized at the electrochemically reactive surfaces of the sensor, while other interferents are known to interfere with the ability of the enzyme (e.g., glucose oxidase) used to react with the analyte being measured. Yet other interferents are known to react with the enzyme (e.g., glucose oxidase) to produce a byproduct that is electrochemically active. Interferents can exaggerate or mask the response signal, thereby leading to false or misleading results. For example, a false positive signal may cause the host's analyte concentration (e.g., glucose concentration) to appear higher than the true analyte concentration. False-positive signals may pose a clinically significant problem in some conventional sensors. For example in a severe hypoglycemic situation, in which the host has ingested an interferent (e.g., acetaminophen), the resulting artificially high glucose signal can lead the host to believe that he is euglycemic or hyperglycemic. In response, the host may make inappropriate treatment decisions, such as by injecting himself with too much insulin, or by taking no action, when the proper course of action would be to begin eating. In turn, this inappropriate action or inaction may lead to a dangerous hypoglycemic episode for the host. Accordingly, it is desired that a membrane system can be developed that substantially reduces or eliminates the effects of interferents on analyte measurements. As described in more detail elsewhere herein, it is contemplated that a membrane system having one or more domains capable of blocking or substantially reducing the flow of interferents onto the electroactive surfaces of the electrode may reduce noise and improve sensor accuracy.

With respect to analyte sensors, it is contemplated that a number of types of interferents may cause inaccurate readings. One type of interferents is defined herein as ‘exogenous interferents.’ The term ‘exogenous interferents’ as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and are not to be limited to a special or customized meaning), and refers without limitation to interferents that affect the measurement of glucose and that are present in the host, but that have origins outside of the body, and that can include items administered to a person, such as medicaments, drugs, foods or herbs, whether administered intravenously, orally, topically, etc. By way of example, acetaminophen ingested by a host or the lidocaine injected into a host would be considered herein as exogenous interferents.

Another type of interferents is defined herein as ‘endogenous interferents.’ The term ‘endogenous interferents’ as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and are not to be limited to a special or customized meaning), and refers without limitation to interferents that affect the measurement of glucose and that have origins within the body, and thus includes interferents derived from species or metabolites produced during cell metabolism (e.g., as a result of wound healing). While not wishing to be bound by theory, it is believed that a local buildup of electroactive interferents, such as electroactive metabolites derived from cellular metabolism and wound healing, may interfere with sensor function and cause early intermittent, sedentary noise. Local lymph pooling, when parts of the body are compressed or when the body is inactive, may also cause, in part, this local buildup of interferents (e.g., electroactive metabolites). It should be noted that endogenous interferents may react with the membrane system in ways that are different from exogenous interferents. Endogenous interferents may include but are not limited to compounds with electroactive acidic, amine or sulfhydryl groups, urea (e.g., as a result of renal failure), lactic acid, phosphates, citrates, peroxides, amino acids (e.g., L-arginine), amino acid precursors or break-down products, nitric oxide (NO), NO-donors, NO-precursors, or other electroactive species or metabolites produced during cell metabolism or wound healing, for example.

Noise-Reducing Membrane System

In some embodiments, the continuous sensor may have a bioprotective domain which includes a polymer containing one or more surface-active groups configured to substantially reduce or block the effect or influence of non-constant noise-causing species. In some of these embodiments, the reduction or blocking of the effect or influence of non-constant noise-causing species may be such that the non-constant noise component of the signal is less than about 60%, 50%, 40%, 30%, 20%, or 10% of the total signal. In some embodiments, the sensor may include at least one electrode and electronics configured to provide a signal measured at the electrode. The measured signal can be broken down (e.g., after sensor break-in) into its component parts, which may include but are not limited to a substantially analyte-related component, a substantially constant non-analyte-related component (e.g., constant noise), and a substantially non-constant non-analyte-related component (e.g., non-constant noise). In some of these embodiments, the sensor may be configured such that the substantially non-constant non-analyte-related component does not substantially contribute to the signal for at least about one or two days. In some embodiments, the signal contribution of the non-constant noise may be less than about 60%, 50%, 40%, 30%, 20%, or 10% of the signal (i.e., total signal) over a time period of at least about one day, but in other embodiments, the time period may be at least about two, three, four, five, six, seven days or more, including weeks or months, and the signal contribution of the non-constant noise may be less than about 18%, 16%, 14%, 12%, 10%, 8%, 6%, 5%, 4%, 3%, 2%, or 1%. It is contemplated that in some embodiments, the sensor may be configured such that the signal contribution of the analyte-related component is at least about 50%, 60%, 70%, 80%, 90% or more of the total signal over a time period of at least about one day; but in some embodiments, the time period may be at least about two, three, four, five, six, seven days or more, including weeks or months, and the signal contribution of the analyte-related component may be at least about 50%, 55%, 60%, 65%, 70%, 75%, 80%, 82%, 84%, 86%, 88%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or more.

A signal component's percentage of the total signal can be determined using a variety of methods of quantifying an amplitude of signal components and total signal, from which each component's percent contribution can be calculated. In some embodiments, the signal components can be quantified by comparing the peak-to-peak amplitudes of each signal component for a time period, whereby the peak-to-peak amplitudes of each component can be compared to the peak-to-peak amplitude of the total signal to determine its percentage of the total signal. In some embodiments, the signal components can be quantified by determining the Root Mean Square (RMS) of the signal component for a time period. In one exemplary of Root Mean Square analysis of signal components, the signal component(s) can be quantified using the formula:

${RMS} = \sqrt{\frac{\sum\left( {x_{1}^{2} + x_{2}^{2} + x_{3}^{2} + x_{n}^{2}} \right)}{n}}$

wherein there are a number (n) of data values (x) for a signal (e.g., analyte component, non-constant noise component, constant noise component, and total signal) during a predetermined time period (e.g., about 1 day, about 2 days, about 3 days, etc.). Once the signal components and total signal are quantified, the signal components can be compared to the total signal to determine a percentage of each signal component within the total signal.

Bioprotective Domain

The bioprotective domain is the domain or layer of an implantable device configured to interface with (e.g., contact) a biological fluid when implanted in a host or connected to the host (e.g., via an intravascular access device providing extracorporeal access to a blood vessel). As described above, membranes of some embodiments may include a bioprotective domain 46 (see FIGS. 2A-2C), also referred to as a bioprotective layer, including at least one polymer containing a surface-active group. In some embodiments, the surface-active group-containing polymer is a surface-active end group-containing polymer. In some of these embodiments, the surface-active end group-containing polymer is a polymer having covalently bonded surface-active end groups. However, it is contemplated that other surface-active group-containing polymers may also be used and can be formed by modification of fully-reacted base polymers via the grafting of side chain structures, surface treatments or coatings applied after membrane fabrication (e.g., via surface-modifying additives), blending of a surface-modifying additive to a base polymer before membrane fabrication, immobilization of the surface-active-group-containing soft segments by physical entrainment during synthesis, or the like.

Base polymers useful for certain embodiments may include any linear or branched polymer on the backbone structure of the polymer. Suitable base polymers may include, but are not limited to, epoxies, polyolefins, polysiloxanes, polyethers, acrylics, polyesters, carbonates, and polyurethanes, wherein polyurethanes may include polyurethane copolymers such as polyether-urethane-urea, polycarbonate-urethane, polyether-urethane, silicone-polyether-urethane, silicone-polycarbonate-urethane, polyester-urethane, and the like. In some embodiments, base polymers may be selected for their bulk properties, such as, but not limited to, tensile strength, flex life, modulus, and the like. For example, polyurethanes are known to be relatively strong and to provide numerous reactive pathways, which properties may be advantageous as bulk properties for a membrane domain of the continuous sensor.

In some embodiments, a base polymer synthesized to have hydrophilic segments may be used to form the bioprotective layer. For example, a linear base polymer including biocompatible segmented block polyurethane copolymers comprising hard and soft segments may be used. In some embodiments, the hard segment of the copolymer may have a molecular weight of from about 160 daltons to about 10,000 daltons, and sometimes from about 200 daltons to about 2,000 daltons. In some embodiments, the molecular weight of the soft segment may be from about 200 daltons to about 10,000,000 daltons, and sometimes from about 500 daltons to about 5,000,000 daltons, and sometimes from about 500,00 daltons to about 2,000,000 daltons. It is contemplated that polyisocyanates used for the preparation of the hard segments of the copolymer may be aromatic or aliphatic diisocyanates. The soft segments used in the preparation of the polyurethane may be a polyfunctional aliphatic polyol, a polyfunctional aliphatic or aromatic amine, or the like that may be useful for creating permeability of the analyte (e.g., glucose) therethrough, and may include, for example, polyvinyl acetate (PVA), poly(ethylene glycol) (PEG), polyacrylamide, acetates, polyethylene oxide (PEO), polyethylacrylate (PEA), polyvinylpyrrolidone (PVP), and variations thereof (e.g., PVP vinyl acetate), and wherein PVP and variations thereof may be selected for their hydrolytic stability in some embodiments.

Alternatively, in some embodiments, the bioprotective layer may comprise a combination of a base polymer (e.g., polyurethane) and one or more hydrophilic polymers, such as, PVA, PEG, polyacrylamide, acetates, PEO, PEA, PVP, and variations thereof (e.g., PVP vinyl acetate), e.g., as a physical blend or admixture wherein each polymer maintains its unique chemical nature. It is contemplated that any of a variety of combination of polymers may be used to yield a blend with desired glucose, oxygen, and interference permeability properties. For example, in some embodiments, the bioprotective layer may be formed from a blend of a polycarbonate-urethane base polymer and PVP, but in other embodiments, a blend of a polyurethane, or another base polymer, and one or more hydrophilic polymers may be used instead. In some of the embodiments involving use of PVP, the PVP portion of the polymer blend may comprise from about 5% to about 50% by weight of the polymer blend, sometimes from about 15% to 20%, and other times from about 25% to 40%. It is contemplated that PVP of various molecular weights may be used. For example, in some embodiments, the molecular weight of the PVP used may be from about 25,000 daltons to about 5,000,000 daltons, sometimes from about 50,000 daltons to about 2,000,000 daltons, and other times from 6,000,000 daltons to about 10,000,000 daltons.

Membranes have been developed that are capable of controlling the flux of a particular analyte passing through the membrane. However, it is known that conventional membranes typically lack the capability of substantially reducing or blocking the flux of interferents passing therethrough. From a membrane design perspective, typically as a membrane is made more permeable (i.e., opened up) for an analyte to pass through, this increased permeability of the membrane for the analyte tends to also increase the permeability of interferents. As an example, a conventional membrane that allows for a flux of glucose (with a M.W. of 180 daltons) through the membrane will typically not substantially reduce or block the flux of interferents, such as acetaminophen (with a M.W. of 151.2 daltons) through the membrane. Accordingly, without a mechanism designed to reduce the flux of interferents, large levels of undesirable signal noise may be generated as a result of the interferents passing through the membrane. Advantageously, some embodiments described herein provide a membrane layer that overcomes the above-described deficiencies by providing a mechanism for selectively controlling the flux of a particular analyte, while also substantially reducing or blocking the flux of interferents through the membrane.

While not wishing to be bound by theory, it is believed that in some conventional membranes formed with various segmented block polyurethane copolymers, the hydrophobic portions of the copolymer (e.g., the hard segments) may tend to segregate from the hydrophilic portions (e.g., the soft segments), which in turn, may cause the hydrophilic portions to align and form channels, through which analytes, such as glucose, and other molecules, such as exogenous interferents like acetaminophen, may pass through the bioprotective layer from the distal surface to the proximal surface. While the diffusion of analytes through the bioprotective layer is desired, the diffusion of interferents is generally not. Through experiments, it has been unexpectedly found that the use of PVP blended with a base polymer, such as, silicone-polycarbonate-urethane, may provide the bioprotective layer with the capability of substantially reducing or blocking the flux of various interferents, such as acetaminophen, through the layer. While not wishing to be bound by theory, it is believed that the carbonyl groups of PVP molecules may form hydrogen bonds with various interferents. For example, acetaminophen molecules are known to be capable of hydrogen bonding via their hydroxyl (O—H) and amide (H—N—(C═O)) groups, and thus through these moieties may interact with PVP. Although PVP is described here to provide an example of a hydrophilic polymer capable of providing the hydrogen bonding effects described above, it is contemplated that any of a variety of other hydrophilic polymers known to have strong hydrogen bonding properties may also be used, such as, polyvinyl pyrrolidone-vinyl acetate (PVP-VA), hydroxypropyl cellulose (HPC), hydroxypropyl methylcellulose (HPMC), for example.

In some embodiments, the bioprotective domain is configured to substantially reduce or block the flux of at least one interferent, and exhibits a glucose-to-interferent permeability ratio of approximately 1 to 30, but in other embodiments the glucose-to-interferent permeability ratio (e.g., glucose-to-acetaminophen permeability ratio) may be less than approximately 1 to 1, 1 to 2, 1 to 5, 1 to 10, 1 to 15, 1 to 20, 1 to 35, 1 to 40, 1 to 45, 1 to 50, or 1 to 100. The glucose-to-interferent permeability ratios exhibited by these embodiments are an improvement over conventional polyurethane membranes which typically exhibit glucose-to-interferent permeability ratios (e.g., glucose-to-acetaminophen permeability ratios) greater than 1 to 300. In some embodiments, the equivalent peak glucose response to a 1,000 mg dose of acetaminophen is less than about 100 mg/dL, sometimes less than 80 mg/dL, and sometimes between about 50 mg/dL, and sometimes less than 20 mg/dL.

In some embodiments, the PVP portion of the polymer blend may comprise from about 5% to about 50% by weight of the polymer blend, sometimes from about 15% to 20%, and other times from about 25% to 40%. It is contemplated that PVP of various molecular weights may be used. For example, in some embodiments, the molecular weight of the PVP used may be from about 25,000 daltons to about 5,000,000 daltons, sometimes from about 50,000 daltons to about 2,000,000 daltons, and other times from 6,000,000 daltons to about 10,000,000 daltons.

The term ‘surface-active group’ and ‘surface-active end group’ as used herein are broad terms and are used in their ordinary sense, including, without limitation, surface-active oligomers or other surface-active moieties having surface-active properties, such as alkyl groups, which preferentially migrate towards a surface of a membrane formed there from. Surface-active groups preferentially migrate toward air (e.g., driven by thermodynamic properties during membrane formation). In some embodiments, the surface-active groups are covalently bonded to the base polymer during synthesis. In some embodiments, surface-active groups may include silicone, sulfonate, fluorine, polyethylene oxide, hydrocarbon groups, and the like. The surface activity (e.g., chemistry, properties) of a membrane domain including a surface-active group-containing polymer reflects the surface activity of the surface-active groups rather than that of the base polymer. In other words, surface-active groups control the chemistry at the surface (e.g., the biological contacting surface) of the membrane without compromising the bulk properties of the base polymer. The surface-active groups of some embodiments are selected for desirable surface properties, for example, non-constant noise-blocking ability, break-in time (reduced), ability to repel charged species, cationic or anionic blocking, or the like. In some embodiments, the surface-active groups are located on one or more ends of the polymer backbone, and referred to as surface-active end groups, wherein the surface-active end groups are believed to more readily migrate to the surface of the bioprotective domain/layer formed from the surface-active group-containing polymer in some circumstances.

FIG. 4A is a schematic view of a base polymer 400 having surface-active end groups in one embodiment. In some embodiments, the surface-active moieties 402 are restricted to the termini of the linear or branched base polymer(s) 400 such that changes to the base polymer's bulk properties are minimized. Because the polymers couple by end groups to the backbone polymer during synthesis, the polymer backbone retains its strength and processability. The utility of surface-active end groups is based on their ability to accumulate at the surface of a formed article made from the surface-active end group-containing polymer. Such accumulation is driven by the minimization of interfacial energy of the system, which occurs as a result of it.

FIG. 4B is a schematic view of a bioprotective domain, showing an interface in a biological environment (e.g., interstitial space or vascular space). A surface-active group-containing polymer is shown fabricated as a membrane 46, wherein the surface-active end groups have migrated to the surface of the base polymer. While not wishing to be bound by theory, it is believed that this surface is developed by surface-energy-reducing migrations of the surface-active end groups to the air-facing surface during membrane fabrication. It is also believed that the hydrophobicity and mobility of the end groups relative to backbone groups facilitate the formation of this uniform over layer by the surface-active (end) blocks.

In some embodiments, the bioprotective domain 46 is formed from a polymer containing silicone as the surface-active group, for example, a polyurethane containing silicone end group(s). Some embodiments include a continuous analyte sensor configured for insertion into a host, wherein the sensor has a membrane located over the sensing mechanism, which includes a polyurethane comprising silicone end groups configured to substantially block the effect of non-constant noise-causing species on the sensor signal, as described in more detail elsewhere herein. In some embodiments, the polymer includes about 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, to about 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54% or 55% silicone by weight. In certain embodiments, the silicone (e.g., a precursor such as PDMS) has a molecular weight from about 500 to about 10,000 daltons, or in some embodiments at least about 200 daltons. In some embodiments, the base polymer includes at least about 10% silicone by weight, for example from about 19% to about 40% silicone by weight. These ranges are believed to provide an advantageous balance of noise-reducing functionality, while maintaining sufficient glucose permeability in embodiments wherein the sensor is a glucose sensor, for example.

In some embodiments, the bioprotective domain is formed from a polymer containing fluorine as a surface-active group, for example, a polyurethane that contains a fluorine end groups. In some embodiments, the polymer includes from about 1% to about 25% fluorine by weight. Some embodiments include a continuous analyte sensor configured for insertion into a host, wherein the sensor has a membrane located over the sensing mechanism, wherein the membrane includes a polyurethane containing fluorine surface-active groups, and wherein the membrane is configured and arranged to reduce a break-in time of a sensor as compared to a membrane formed from a similar base polymer without the surface-active group(s). For example, in some preferred embodiments, a glucose sensor having a bioprotective domain has a response time (e.g., t₉₀) of less than 120 seconds, sometimes less than 60 seconds, and sometimes less than about 45, 30, 20, or 10 seconds (across a physiological range of glucose concentration).

In some embodiments, the bioprotective domain may be formed from a polymer that contains sulfonate as a surface-active group, for example, a polyurethane containing sulfonate end group(s). In some embodiments, the continuous analyte sensor configured for insertion into a host may include a membrane located over the sensing mechanism, wherein the membrane includes a polymer that contains sulfonate as a surface-active group, and is configured to repel charged species, for example, due to the net negative charge of the sulfonated groups.

In some embodiments, a blend of two or more (e.g., two, three, four, five, or more) surface-active group-containing polymers is used to form a bioprotective membrane domain. For example, by blending a polyurethane with silicone end groups and a polyurethane with fluorine end groups, and forming a bioprotective membrane domain from that blend, a sensor can be configured to substantially block non-constant noise-causing species and reduce the sensor's t₉₀, as described in more detail elsewhere herein. Similarly, by blending a polyurethane containing silicone end groups, a polyurethane containing fluorine end groups, and a polyurethane containing sulfonate end groups, and forming a bioprotective membrane domain from that blend, a sensor can be configured to substantially block non-constant noise-causing species, to reduce the sensor's break-in time and to repel charged species, as described in more detail above. Although in some embodiments, blending of two or more surface-active group-containing polymers is used, in other embodiments, a single component polymer can be formed by synthesizing two or more surface-active groups with a base polymer to achieve similarly advantageous surface properties; however, blending may be advantageous in some embodiments for ease of manufacture.

Interference Domain

It is contemplated that in some embodiments, such as in the sensor configuration illustrated in FIG. 2B, an interference domain 40, also referred to as the interference layer, may be provided in addition to (or in replacement of) the bioprotective domain. The interference domain 40 may substantially reduce the permeation of one or more interferents into the electrochemically reactive surfaces. The interference domain 40 can be configured to be much less permeable to one or more of the interferents than to the measured species. It is also contemplated that in some embodiments, where interferent blocking may be provided by the bioprotective domain (e.g., via a surface-active group-containing polymer of the bioprotective domain), a separate interference domain is not present. In other embodiments, the membrane includes both an interference domain and a bioprotective domain, with both domains configured to reduce the permeation of one or more interferents. In further embodiments, the interference domain and the bioprotective domain are each configured to reduce permeation of different interfering species. For example, the interference domain may have greater specificity than the bioprotective domain with respect to reducing permeation of one type of interfering species, while the bioprotective domain may have greater specificity than the interference domain with respect to reducing permeation of another type of interfering species. In some embodiments, both the interference domain and the bioprotective domain are configured to target certain interference species for permeation reduction.

In certain embodiments, the implantable sensor employs a membrane system comprising a resistance domain, an enzyme domain, and an interference domain. The interference domain can be proximal to the sensor and the resistance domain can be distal to the sensor, with the enzyme domain therebetween. The interference domain can consist of a single layer or plurality of layers of the same material. However, in some embodiments, the interference domain comprises two or more different types of layers in an alternating configuration. For example, a first type of layer can be represented by X, a second type of layer can be represented by Y, and a third type of layer can be represented by Z. The interference domain including alternating layers can have the following exemplary configurations:

XY YX XYX XYXYX XYXYXY XXYYXYXXYY XXXYYYXXXYYYXXX XYXYXYXYXYXYX XYZXYZXYZX XYXZXYXZXYXZ ZYYXZZZXYYYXZ

The above configurations, which are merely exemplary, illustrate various embodiments. In certain embodiments, the first and last layers are the same (e.g., X and X), in other embodiments, the first and last layers are different (e.g., X and Y). The domain can include one or more layers that are unitary (i.e., a single layer is deposited, e.g., X), or composite (e.g., a first layer of material is deposited, followed by the deposition of a second and third, etc. layer of the same material atop the first layer, e.g., XXX). The pattern of alternating layers can be regular (e.g., XYXYXYXYXY) or irregular (e.g., ZYXZXYZYZ).

In some embodiments, the alternating layers include polyanionic layers and polycationic layers. The following are exemplary interference domain configurations, wherein the polyanionic layers (unitary, composite, and/or contiguous with the same polyanion or with different polyanions) are represented by A and the polycationic layers by C (unitary, composite, and/or contiguous with the same polyanion or with different polyanions):

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Other configurations (e.g., those including additional layers, and/or additional materials) are also contemplated for some embodiments. In some embodiments, each A layer is a unitary or composite layer of the same polyanion, and each C layer is a unitary or composite layer of the same polycation. The outermost layers of the interference domain can both be polycation layers, with polyanion layers present only as interior layers. Any suitable number of alternating layers can be employed in the interference domain, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 or more bilayers (defined as a polycationic layer adjacent to a polyanionic layer). In some embodiments a final polycationic layer is added so as to yield an interference domain with polycationic layers as the outermost layers. In other embodiments a final anionic layer is added so as to yield an interference domain with polyanionic layers as the outermost layers.

Polyanions and polycations belong to the class of polymers commonly referred to as polyelectrolytes-polymers wherein at least some of the repeating units (or monomers) include one or more ionic moieties. Polyelectrolytes which bear both cationic and anionic moieties are commonly referred to as polyampholytes. Certain polyelectrolytes form self-assembled monolayers wherein one end of the molecule shows a specific, reversible affinity for a substrate such that an organized, close-packed monolayer of the polyelectrolyte can be deposited.

The polycation can be any biocompatible polycationic polymer. In some embodiments, the polycation is a biocompatible water-soluble polycationic polymer. In certain embodiments, water solubility may be enhanced by grafting the polycationic polymer with water-soluble polynonionic materials such as polyethylene glycol. Representative polycationic materials may include, for example, natural and unnatural polyamino acids having a net positive charge at neutral pH, positively charged polysaccharides, and positively charged synthetic polymers. Additional examples of suitable polycationic materials include polyamines having amine groups on either the polymer backbone or the polymer sidechains, such as poly-L-lysine and other positively charged polyamino acids of natural or synthetic amino acids or mixtures of amino acids, including poly(D-lysine), poly(ornithine), poly(arginine), and poly(histidine), and nonpeptide polyamines such as poly(aminostyrene), poly(aminoacrylate), poly(N-methyl aminoacrylate), poly (N-ethylaminoacrylate), poly(N,N-dimethyl aminoacrylate), poly(N,N-diethylaminoacrylate), poly(diallyldimethyl ammonium chloride), poly(aminomethacrylate), poly(N-methyl amino-methacrylate), poly(N-ethyl aminomethacrylate), poly(N,N-dimethyl aminomethacrylate), poly(N,N-diethyl aminomethacrylate), poly(ethyleneimine), polymers of quaternary amines, such as poly(N,N,N-trimethylaminoacrylate chloride), poly(methyacrylamidopropyltrimethyl ammonium chloride), and natural or synthetic polysaccharides such as chitosan, poly(allylamine hydrochloride), poly(diallyldimethylammonium chloride), poly(vinylbenzyltriamethylamine), polyaniline or sulfonated polyaniline, (p-type doped), polypyrrole (p-type doped), polyallylamine gluconolactone, and poly(pyridinium acetylene).

The polyanionic material can be any biocompatible polyanionic polymer, for example, any polymer having carboxylic acid groups attached as pendant groups. The polyionic layers can be hydrophilic (e.g., a material or portion thereof which will more readily associate with water than with lipids). In some embodiments, the polyanionic polymer is a biocompatible water-soluble polyanionic polymer. Suitable materials include, but are not limited to, alginate, carrageenan, furcellaran, pectin, xanthan, hyaluronic acid, heparin, heparan sulfate, chondroitin sulfate, dermatan sulfate, dextran sulfate, polymethacrylic acid, polyacrylic acid, poly(vinyl sulfate), poly(thiophene-3-acetic acid), poly(4-styrenesulfonic acid), poly(styrene sulfonate), (poly[1-[4-(3-carboxy-4-hydroxy-phenylazo)benzene sulfonamido]-1,2-ethanediyl, sodium salt]), poly(4-[4-({4-[3-amino-2-(4-hydroxy-phenyl)propylcarbamoyl]-5-oxo-pentyl-}-methyl-amino)-phenylazo]-benzenesulfonic acid), oxidized cellulose, carboxymethyl cellulose and crosmarmelose, synthetic polymers, and copolymers containing pendant carboxyl groups, such as those containing maleic acid or fumaric acid in the backbone. Polyaminoacids of predominantly negative charge are also suitable. Examples of these materials include polyaspartic acid, polyglutamic acid, and copolymers thereof with other natural and unnatural amino acids. Polyphenolic materials, such as tannins and lignins, can be used if they are sufficiently biocompatible.

The molecular weight of the polyionic materials may be varied in order to alter coating characteristics, such as coating thickness. As the molecular weight is increased, the coating thickness generally increases. However, an increase in molecular weight may result in greater difficulty with handling. To achieve a balance of coating thickness, material handling, and other design considerations, the polyionic materials can have a particular average molecular weight Mn. In some embodiments, the average molecular weight of a polyionic material used is from about 1,000, 10,000, or 20,000 to about 25,000, 50,000, 100,000 or 150,000 g/mol.

In some embodiments, the interference domain can be prepared using a layer-by-layer deposition technique, wherein a substrate (e.g., the sensor or membrane layer atop the sensor, e.g., the resistance or enzyme layer) is dipped first in a bath of one polyelectrolyte, then in a bath of an oppositely charged polyelectrolyte. Optionally, the substrate can be dipped in a bath of rinsing solution before or after the substrate is dipped into the polyelectrolyte bath. During each dip a small amount of polyelectrolyte is adsorbed and the surface charge is reversed, thereby allowing a gradual and controlled build-up of electrostatically cross-linked films (or hydrogen bonded films) of alternating polycation-polyanion layers. The method provides a technique for controlling functionality and film thickness and functionality. For example, it can be employed for depositing films as thin as one monolayer or for thicker layers. FIG. 9B illustrates one embodiment of a layer-by-layer deposition method, which employs alternating adsorption of polycations and polyanions to create a structure illustrated in FIG. 9A. Operationally, the embodiment illustrated in FIG. 9B occurs through consecutive exposures of a substrate 938 to polycation and polyanion solutions, with rinsing to remove unadsorbed polymer after each deposition step. In a first step, a polycation 942 is deposited onto a substrate 938 (e.g., a wire with an electroactive surface or a flat wafer substrate) to form a polycationic layer 942. As described elsewhere herein in greater detail, the deposition of the layer can be performed using any of a variety of techniques, such as, dipping and/or spraying, for example. In a second step, rinsing is performed to remove unadsorbed polymer after deposition of the polycationic layer 942. Next, in a third step, a polyanion 944 is deposited onto the polycationic layer 942. Thereafter, in a fourth step, rinsing is performed to remove unadsorbed polymer after deposition of the polyanionic layer 944. These steps can be repeated until the desired interference domain configuration and/or structure is achieved. In an alternative embodiment, instead of depositing a polycationic layer as the first layer on top of the substrate 938, a polyanionic layer is deposited instead. Thereafter, a second layer formed of a polycation is deposited onto the first layer, i.e., the polyanionic layer. This process is continued until a certain desired interference domain configuration and/or structure is achieved.

In some embodiments, methods can also employ other interactions such as hydrogen bonding or covalent linkages. Depending upon the nature of the polyelectrolyte, polyelectrolyte bridging may occur, in which a single polyelectrolyte chain adsorbs to two (or more) oppositely charged macroions, thereby establishing molecular bridges. If only a monolayer of each polyelectrolyte adsorbs with each deposition step, then electrostatically cross-linked hydrogel-type materials can be built on a surface a few microns at a time. If the substrate is not thoroughly rinsed between the application of polyionic films, thicker, hydrogel-like structures can be deposited.

In some embodiments, the interference blocking ability provided by the alternating polycationic layer(s) and polyanionic layer(s) can be adjusted and/or controlled by creating covalent cross-links between the polycationic layer(s) and polyanionic layer(s). Cross-linking can have a substantial effect on mechanical properties and structure of the film, which in turn can affect the film's interference blocking ability. Cross-linked polymers can have different cross-linking densities. In certain embodiments, cross-linkers are used to promote cross-linking between layers. In other embodiments, in replacement of (or in addition to) the cross-linking techniques described above, heat is used to form cross-linking. For example, in some embodiments imide and amide bonds can be formed between a polycationic layer and a polyanionic layer as a result of high temperature. In some embodiments, photo cross-linking is performed to form covalent bonds between the polycationic layers(s) and polyanionic layer(s). One major advantage to photo-cross-linking is that it offers the possibility of patterning. In certain embodiments, patterning using photo-cross linking is performed to modify the film structure and thus to adjust the interference domain's interference blocking ability. Blocking ability can correspond to, but is not limited to, the ability to reduce transport of a certain interfering species or to the selectivity for the transport of a desired species (e.g., H₂O₂) over an interfering species. Post-deposition reactions, such as cross-linking and reduction of metal ions to form nanoparticles, provide further ways to modify film properties. In some embodiments, cross-linking may be performed between deposition of adjacent polycationic or polyanionic layers in replacement of (or in addition to) a post-deposition cross-linking process.

The overall thickness of the interference layer can impact its permeability to interferents. The overall thickness of the interference domain can be controlled by adjusting the number of layers and/or the degree of rinsing between layers. With layer deposition through spraying, control of drop size and density can provide coatings of desired selected thickness without necessarily requiring rinsing between layers. Additionally, the excess (unbound) material can be removed via other means, for example, by an air jet. If the residual polyelectrolyte from the previous layer is substantially removed before adding the subsequent layer, the thickness per layer decreases. Accordingly, in one embodiment, the surface is first coated with a polycation, the excess polycation is then removed by rinsing the surface, afterwards the polyanion is added, the excess is then removed, and the process is repeated as necessary. In some embodiments, the polycations or polyanions from different adjacent layers may intertwine. In further embodiment, they may be intertwined over several layers.

In some embodiments, the level of ionization of polyions may be controlled, for example, by controlling the pH in the dip solution comprising the polycation or the polyanion. By changing the level of ionization of these polyions, the interference blocking ability of a certain layer of may be altered and/or controlled. For example, a first polycationic layer that has a higher level of ionization than a second polycationic layer may be better at interacting with and reducing the transport a first interfering species, while the second polycationic may be better at interacting with and reducing the transport of a second interfering species. Changes in the level of ionization of a polyion's charge groups can also affect the mechanical properties, structural properties, and other certain properties (e.g., diffusion properties) that may affect the interference domain's ability to reduce transport of (or entirely block) interfering species. For example, an alternating bilayer, comprising polycations and polyanions, both of which have high levels of ionization, may bond together more tightly than a corresponding bilayer with low levels of ionization. Thus, the structural difference between these two membranes, which can be in the form of mechanical properties or other properties (e.g., thickness of the domain), can affect the performance of the interference domain.

In some embodiments, the linear charge density of the polyelectrolyte may be controlled at least in part by the average charge spacing along the polyion chain. The spacing between charge groups on the polycationic and/or polyanionic polymers that form the interference domain may be controlled by polyelectrolyte polymer selection or polymer synthesis. How far the charged groups are spaced can greatly affect the structural properties of the interference domain. For example, a polyion having charged groups that are spaced closely to each other may result in small-sized pores in the interference domain, thereby resulting in a structure that excludes medium molecular-sized and large molecular-sized interfering species from passage therethough, while allowing passage therethrough of small-sized pores. Conversely, a polyion having charged groups that are spaced apart at a moderate distance from each other may result in medium-sized pores that exclude large molecular-sized interfering species and allow passage therethrough of medium-molecular sized and small molecular-sized interfering species. In certain embodiments, the linear charge density of the polyanionic polymer is from about 1 to 50 e/Å, sometimes from about 10 to 25 e/Å, sometimes from about 2 to 10 e/Å, and sometimes from 2 to 3 e/Å, where e is the elementary charge of an electron/proton and Å is distance in angstroms. In some embodiments, the linear charge density of the polycationic polymer is from about 1 to 50 e/Å, sometimes from about 10 to 15 e/Å, sometimes from about 2 to 10 e/Å, and sometimes from 2 to 3 e/Å.

In some embodiments, the linear charge density of polyanionic polymer is substantially similar to the linear charge density of the polycationic polymer. For example, in one embodiment, the polyanionic layer is formed of (i) poly(acrylic acid), which has an average linear charge density of about 2.5 e/Å and (ii) poly(allylamine hydrochloride), which also has an average linear charge density of about 2.5 e/Å. In certain embodiments, the polycationic and polyanionic layers may have an average linear charge density that is substantially equal with each other and that is from about 1 to 50 e/Å, sometimes from about 2 to 25 e/Å, sometimes from about 5 to 10 e/Å, other times from about 10 to 15 e/Å, and other times from about 15 to 25 e/Å.

By providing an interference domain with differing linear charge densities, an interference domain may be formed that comprises different polycationic/polyanionic bilayers that are designed specifically to exclude different interfering species based on certain characteristics (e.g., molecular size) of the targeted interfering species. For example, in one embodiment, an outermost bilayer of the interference domain is designed to have a medium average charge spacing, thereby resulting in a bilayer that only excludes large molecular-sized species, but allows passage therethrough of medium molecular-sized species and small molecular-sized species. Conversely, an innermost bilayer of the interference domain may be designed to have low average charge spacing, thereby resulting in a bilayer that excludes all molecules, except those with very small molecular sizes, for example, H₂O₂.

In some embodiments, the polycationic layers may be formed of the same or substantially the same material (e.g., poly(allylamine hydrochloride) (PAH) for polycation or poly(acrylic acid) (PAA) for polyanion), while having different levels of ionization. For example, in one embodiment, the interference domain comprises seven alternating polyelectrolyte layers, with the first, third, fifth, and seventh layers being polycationic layers, and with the second, fourth, and sixth layers being polyanionic layers, wherein the first and seventh layers form the outer layers of the interference domain. In one embodiment, each or some of the polycationic layers may have different levels of ionization. For example, in one embodiment, the first, third, fifth, and seventh layers may each have different levels of ionization, with the first layer having the highest level of ionization and the seventh layer having the lowest level of ionization, or vice versa. In an alternative embodiment, some of the polycationic layers may share substantially the same level of ionization. For example, in one embodiment, the first and seventh layers may have substantially the same levels of ionization, while the third and fifth layers may have a level of ionization that is different from the others. As described elsewhere herein, the ionization level of a polyion may be controlled by controlling the pH in the dip solution comprising the polycation or the polyanion. By changing the level of ionization of these polyions, the interference blocking ability of a certain layer of may be altered and/or controlled.

The design of an interference domain having layers with levels of ionization can also be applied to polyanionic layers as well. For example, in one embodiment with seven alternating polyelectrolyte layers, the second, fourth, and sixth layers are each polyanionic layers and may each have different levels of ionization, with the second layer having the highest level of ionization and the sixth layer having the lowest level of ionization, or vice versa. In an alternative embodiment, some of the polyanionic layers may share substantially the same level of ionization. For example, in one embodiment, the second and fourth layers may have substantially the same levels of ionization, while the sixth layer may have a substantially different level of ionization from the others.

In certain embodiments, the particular polycationic layer(s) and/or polyanionic layer(s) selected to form the interference layer may depend at least in part on their ability to block, reduce, or impede passage therethrough of one or more interferents. For example, the polyanionic layer can be selected for its ability to block, reduce, or impede passage of a first interferent, whereas the polycationic layer is selected for its ability to block, reduce, or impede passage of a second interferent. The layer may be designed to slow but not block passage of an interferent therethrough, or designed to substantially block (e.g., trap) an interferent therein. Additional polyionic layers can be included in the interference domain with particular selectivity towards still different interferents. Depending upon the position of the interference domain in the membrane system relative to the electrode or electroactive surface of the sensor, the permeability of the layer to substances other than the interferent can be important. In sensor systems wherein H₂O₂ (hydrogen peroxide) is produced by an enzyme-catalyzed reaction of an analyte being detected, the interference domain should be designed to allow H₂O₂ to pass through with minimal impedance if the interference domain is positioned between the electroactive surface and the enzyme layer. On the other hand, if in a different membrane design, the interference domain is positioned distal to the enzyme layer (with respect to the electroactive surface), then in some embodiments, the interference domain may be designed to block H₂O₂ not produced by the enzyme-catalyzed reaction from passing therethrough. In addition, with this particular membrane design, the interference domain may be configured to allow analyte and oxygen to pass therethrough with minimal impedance.

Application of the layers in forming the interference domain may be accomplished by various methods known in the art. One coating process embodiment involves solely dip-coating and dip-rinsing steps. Another coating process embodiment involves solely spray-coating and spray-rinsing steps. However, a number of alternative embodiments involve various use of a combination of spray-coating, dip-coating, and/or rinsing steps. For example, one dip-coating method involves the steps of applying a coating of a first polyionic material to a substrate (e.g., the sensor or membrane layer atop the sensor, e.g., the resistance or enzyme layer) by immersing the substrate in a first solution of a first polyionic material; rinsing the substrate by immersing the substrate in a rinsing solution; and, optionally, drying the substrate. This procedure is then repeated using a second polyionic material, with the second polyionic material having charges opposite of the charges of the first polyionic material, in order to form a polyionic bilayer. This bilayer formation process can be repeated a plurality of times in order to produce the interference domain. In some embodiments, the number of bilayers can be from 1 to about 16 bilayers, sometimes from 1 to about 10 bilayers, and sometimes from about 3 to about 7 bilayers. In certain embodiments, the final layer of oppositely charged polyionic material can be deposited, such that the first and the last layer have the same charges (both positive, or both negative). The immersion time for each of the coating and rinsing steps may vary depending on a number of factors. For example, immersion of the substrate into the polyionic solution can occur over a period of about 1 to 30 minutes, or from about 2 to 20 minutes, or from about 1 to 5 minutes. Rinsing may be accomplished in one step, but a plurality of rinsing steps can also be employed. Rinsing in a series from about 2 to 5 steps can be employed, with each immersion into the rinsing solution consuming, for example, from about 1 to about 3 minutes. In some embodiments, several polycationic solutions and/or several polyanion solutions may be used. For example, in certain embodiments, the dip-coating sequence may involve the steps of applying a coating of a first polycationic material to the substrate to form a first layer, then applying a first anionic material to the first layer to form a second layer, then applying a second polycationic material to the second layer to form a third layer, then applying a second polyanionic material to form a fourth layer, and then applying a first or second polycationic material to the fourth layer to form a fifth layer. In some of these embodiments, the dip-coating sequence described above may be interspersed with rinsing steps performed between coating steps. It is contemplated that any of a variety of permutations involving the steps and materials described may be employed. In alternative embodiments, the materials used to form the polycationic and/or polyanionic layers may be substantially the same. However, the individual polycationic layers may have a different level of ionization than one or more other polycationic layers in the inference domain, and the individual polyanionic layers may also have a different level of ionization than one or more other polyanionic layers. For example, in one embodiment, the dip-coating sequence method involves the use of a first solution at a first pH comprising a polycationic material, a second solution at a second pH comprising a polyanionic material, a third solution at a third pH comprising the aforementioned polycationic material, a fourth solution at a fourth pH comprising the aforementioned polyanionic material, and a fifth solution at a fifth pH comprising the aforementioned polycationic material. Even though the same polycationic material is used to form the first, third, and fifth layers, because the solution used to form the first, third, and fifth layers have different pHs, the ionization levels of the first, third, and fifth layers will be different. Likewise, even though the same polyanionic material is used to form the second and fourth layers, because the solution used to form the second and fifth layers have different pHs, the levels of ionization of the second and fourth layers will be different. This difference in ionization levels can affect, inter alia, the mechanical properties of the film, structural properties (e.g., porosity, roughness) of the film, diffusional properties of the film, and also the selectivity of a certain polyelectrolyte layer for a certain interfering species over another interfering species. All of these effects influence the ability of the individual polyelectrolyte layers and of the interference domain to reduce transport of a variety of interfering species. In certain embodiments, at least two polycationic and/or two polyanionic layers of the interference domain are formed from the same polycationic/polyanionic material, but through use of solutions at different pHs. In some of these embodiments, a first polycationic layer possesses a high selectivity for a particular interfering species over other interfering species, while a second polycationic layer possesses a high selectivity for a different interfering species over other interfering species.

Alternatively or additionally, spray coating techniques can be employed. In one embodiment, the coating process generally includes the steps of applying a coating of: a first polyionic material to the substrate by contacting the substrate with a first solution of a first polyionic material; rinsing the substrate by spraying the substrate with a rinsing solution; and (optionally) drying the substrate. Similar to the dip-coating process, the spray-coating process may then be repeated with a second polyionic material, with the second polyionic material having charges opposite to those of the first polyionic material. The contacting of the substrate with solution, either polyionic material or rinsing solution, may occur through a variety of methods. For example, the substrate may be dipped into both solutions. One alternative is to apply the solutions in a spray or mist form. Of course, various combinations are possible and within the scope of the contemplated embodiments, e.g., dipping the substrate in the polyionic material followed by spraying the rinsing solution. The spray coating application may be accomplished via a number of methods known in the art. For example, a conventional spray coating arrangement may be used, i.e., the liquid material is sprayed by application of fluid, which may or may not be at an elevated or lowered pressure, through a reduced diameter nozzle which is directed towards the deposition target. Another spray coating technique involves the use of ultrasonic energy, whereby the liquid is atomized by the ultrasonic vibrations of a spray forming tip and thereby changed to a spray.

Yet another technique involves electrostatic spray coating in which a charge is conveyed to the fluid or droplets to increase the efficiency of the coating. A further method of atomizing liquid for spray coating involves purely mechanical energy, e.g., through contacting the liquid with a high speed reciprocating member or a high speed rotating disk. Still another method of producing microdroplets for spray coatings involves the use of piezoelectric elements to atomize the liquid. These techniques can be employed with air assistance or at an elevated solution pressure. In addition, a combination of two or more techniques may prove more useful with certain materials and conditions. A method of spray application involves dispensing, with a metering pump, the polyanion or polycation solution to an ultrasonic dispensing head. The polyion layer is sprayed so as to allow the surface droplets to coalesce across the material surface. The resulting layer may then be allowed to interact for a period of time or immediately rinsed with water or saline solution (or other solution devoid of polyanion or polycation).

In some embodiments, the layers of the interference domain can include a polymer with a conjugated pi system. Polymers with conjugated pi systems can contain a delocalized electron system, and can be conductive. Layers of polymers with conjugated pi systems can interact with each other through intermolecular forces, such as electrostatic pi-pi interactions (i.e., pi-stacking). Conjugated polymers can provide beneficial properties to an interference domain, such as increasing the rigidity, integrity, and/or reproducibility of the domain. In some embodiments, the polymer with a conjugated pi system can be polyacetylene, polypyrrole, polythiophene, poly(p-phenylene), poly(p-phenylenevinylene) or poly(carbazole). The interference domain can include alternating layers of any of the conjugated polymers mentioned above. In some embodiments, the number of layers of conjugated polymers can be from 1 to about 20 layers, sometimes from about 3 to about 10 layers.

It is contemplated that in some embodiments, the thickness of the interference domain may be from about 0.01 microns or less to about 20 microns or more. In some of these embodiments, the thickness of the interference domain may be from about 0.01, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1, 1.5, 2, 2.5, 3, or 3.5 microns to about 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 19.5 microns. In some of these embodiments, the thickness of the interference domain may be from about 0.2, 0.4, 0.5, or 0.6, microns to about 0.8, 0.9, 1, 1.5, 2, 3, or 4 microns.

Polyimine Films

In some embodiments, certain polymeric films can be used to form interference domains. For example, certain polyimides prepared from 2,2′-dimethyl-4,4′-diaminobiphenyl and the corresponding dianhydride can be cast into films that can be employed as hydrogen peroxide-selective membranes. See, e.g., Ekinci et al., Turk. J. Chem. 30 (2006), 277-285. In one embodiment, a film is prepared using the following steps. First, n-methyl-2-pyrrolidene (NMP) is distilled over CaH₂ under reduced pressure and is stored over about 4 Å molecular sieves. Reagent grade pyromellitic dianhydride (PMDA) is sublimed at about 250° C. under reduced pressure and dried under vacuum at about 120° C. prior to use. The diamine is purified via recrystallization from ethanol to give shiny crystals. Next, 2,20-dimethyl-4,40-diaminobiphenyl, (about 1.06 g, about 5 mmol) is dissolved in NMP (about 15 mL) in a 50 mL Schlenk tube equipped with a nitrogen line, overhead stirrer, a xylene filled Dean-Stark trap, and a condenser. PMDA (about 1.09 g, about 5 mmol) is then added to the amine solution, followed by overnight stirring resulting in a viscous solution. After being stirred for about 3 hours, the solution is heated to reflux at about 200° C. for about 15 hours. During the polymerization process, the water generated from the imidization is allowed to distill from the reaction mixture together with about 1-2 mL of xylene. After being allowed to cool to ambient temperature, the solution is diluted with NMP and then slowly added to a vigorously stirred solution of 95% ethanol. The precipitated polymer is collected via filtration, washed with ethanol, and dried under reduced pressure at 150° C. Before coating, a substrate (e.g., Pt electrode) is cleaned and optionally polished with aqueous alumina slurry down to about 0.05 Then about 20 μL of polymer solution prepared by dissolving about 70 mg of polyimide in about 2 mL of NMP is dropped onto the surface of the Pt electrode and allowed to dry at room temperature for about 3 days.

Self Assembly Techniques

A self-assembly process can be employed to build up ultrathin multilayer films comprising consecutively alternative anionic and cationic polyelectrolytes on a charged surface. See, e.g., Decher et al., Thin Solid Films, 210-211 (1992) 831-835. Ionic attraction between opposite charges is the driving force for the multilayer buildup. In contrast to chemisorption techniques that require a reaction yield of about 100% in order to maintain surface functional density in each layer, no covalent bonds need to be formed with a self-assembly process. Additionally, an advantage over the classic Langmuir-Blodgett technique is that a solution process is independent of the substrate size and topology. Exemplary polyelectrolytes for use in such a process include, but are not limited to, polystyrenesulfonate sodium salt, polyvinylsulfate potassium salt, poly-4-vinylbenzyl-(N,N-diethyl-N-methyl-)-ammonium iodide, and poly(allylamine hydrochloride). The buildup of multilayer films can be conducted as follows. A solid substrate with a positively charged planar surface is immersed in the solution containing the anionic polyelectrolyte and a monolayer of the polyanion is adsorbed. Since the adsorption is carried out at relatively high concentrations of polyelectrolyte, a number of ionic groups remain exposed to the interface with the solution and thus the surface charge is reversed. After rinsing in pure water the substrate is immersed in the solution containing the cationic polyelectrolyte. Again a monolayer is adsorbed but now the original surface charge is restored. By repeating both steps in a cyclic fashion, alternating multilayer assemblies of both polymers are obtained. This process of multilayer formation is based on the attraction of opposite charges, and thus requires a minimum of two oppositely charged molecules. Consequently, one is able to incorporate more than two molecules into the multilayer, simply by immersing the substrate in as many solutions of polyeletrolytes as desired, as long as the charge is reversed from layer to layer. Even aperiodic multilayer assemblies can easily be prepared. In this respect, the technique is more versatile than the Langmuir-Blodgett technique which is rather limited to periodically alternating layer systems. Another advantage is that the immersion procedure does not pose principal restrictions as to the size of the substrate or to the automation in a continuous process.

Specific examples of the preparation of such films are as follows. Polystyrenesulfonate (sodium salt, Mr=100,000) and polyvinylsulfate (potassium salt, Mr=245,000) and poly(allylamine hydrochloride), Mw=50,000-65,000) are obtained from commercial sources and employed without further purification. Poly-4-vinylbenzyl-(N,N-diethyl-N-methyl-)-ammonium iodide can be synthesized, as described in Decher et al., Ber. Bunsenges. Phys. Chem., 95 (1992) 1430. Alternating multilayer assemblies of all materials can be characterized by UV/vis spectroscopy and small angle X-ray scattering (SAXS) using techniques known in the art. Direct-light microscopy and SAXS measurements can be performed with multilayer assemblies on suitable substrates. The multilayer films can be deposited on, e.g., atop a platinum electrode or other metal electrode, or a suitable intervening layer atop an electrode. For the adsorption of the first layer, an aqueous acidic solution of polystyrenesulfonate or polyvinylsulfate can be used. Afterwards the substrate is rinsed with water. After the adsorption of the first layer, the substrates can be stored for some weeks without noticeable deterioration of the surface. Thereafter, the cationic polyelectrolyte polyallylamine is adsorbed from aqueous solution. In the case of the non-quarternized polyallylamine, the polycation is adsorbed from an acidic solution. All following layers (odd layer numbers) of the anionic polyelectrolytes are adsorbed from aqueous solution. In the case of samples containing polyallylamine as the previously adsorbed layer, polystyrenesulfonate layers can be adsorbed from an acidic solution. An adsorption time of about 20 minutes at ambient temperature can be employed, however, in certain embodiments longer or shorter adsorption times may be acceptable. A range of polymer concentrations (e.g., 20 to 30 mg per about 10 ml water) can provide acceptable results.

Multilayer molecular films of polyelectrolyte:calixarene and polyelectrolyte:cyclodextrin hosts can be fabricated by alternating adsorption of charged species in aqueous solutions onto a suitable substrate. See, e.g., X. Yang, Sensors and Actuators B 45 (1997) 87-92. Such a layer-by-layer molecular deposition approach can be used to integrate molecular recognition reagents into polymer films. The deposition process is highly reproducible and the resulting films are uniform and stable. Replacing polyanions, highly negatively charged molecular species can be used for film fabrication. These molecular reagents are capable of binding organic species and can be deposited as functional components into thin films. This approach incorporates polymer and molecular elements into the film and thus results in films with polymer's physical properties and molecular film's selectivity. Films can be prepared as follows. The substrate (e.g., Pt electrode) can be first treated with aminopropyltrimethoxysilane in chloroform, followed with deposition of PSS and then PDDA polyelectrolytes by dipping into the aqueous solutions of the polyelectrolytes, respectively. After this, alternating depositions of negatively charged molecular host species (e.g., calix[6]arene or p-t-butylcalix[4]arene) and PDDA can be carried out until the desired number of bilayers is reached. Between each deposition, the substrate is thoroughly rinsed with deionized water. The polyelectrolyte and molecular ion assembly can be monitored by UV-vis absorption spectroscopy and mass loading can be measured with surface acoustic wave (SAW) devices.

Polyurethane Membranes

Hydrophobic-hydrophilic copolymer films such as are described in U.S. Patent Publication No. US-2006-0086624-A1 can advantageously be employed in sensors of preferred embodiments. Films comprising the copolymer can be prepared as follows. A coating solution is prepared by placing approximately 281 gm of dimethylacetamide (DMAC) into a 3 L stainless steel bowl to which a solution of polyetherurethaneurea (344 gm of Chronothane H (Cardiotech International, Inc., Woburn, MA), 29,750 cp @25% solids in DMAC) is added. To this mixture is added another polyetherurethaneurea (approximately 312 gm, Chronothane 1020 (Cardiotech International, Inc., Woburn, MA), 6275 cp @25% solids in DMAC). The bowl is then fitted to a planetary mixer with a paddle-type blade and the contents are stirred for 30 minutes at room temperature. Coatings solutions prepared in this manner are then coated at between room temperature to about 70° C. onto a substrate, e.g., using a knife-over-roll set at a 0.012 inch gap. The film is continuously dried at 120° C. to about 150° C. The final film thickness is approximately 0.0015 inches.

Membrane Fabrication

Polymers of the various embodiments may be processed by solution-based techniques such as spraying, dipping, casting, electrospinning, vacuum deposition, vapor deposition, spin coating, and the like. Water-based polymer emulsions can be fabricated to form membranes by methods similar to those used for solvent-based materials. In both cases, the evaporation of a volatile liquid (e.g. organic solvent or water) leaves behind a film of the polymer. Cross-linking of the deposited film may be performed through the use of multi-functional reactive ingredients by a number of methods well known to those skilled in the art. The liquid system may cure by heat, moisture, high-energy radiation, ultraviolet light, or by completing the reaction, which produces the final polymer in a mold or on a substrate to be coated.

Domains that include at least two surface-active group-containing polymers may be made using any of the methods of forming polymer blends known in the art. In one exemplary embodiment, a solution of a polyurethane containing silicone end groups is mixed with a solution of a polyurethane containing fluorine end groups (e.g., wherein the solutions include the polymer dissolved in a suitable solvent such as acetone, ethyl alcohol, DMAC, THF, 2-butanone, and the like). The mixture can then be drawn into a film or applied to a surface using any method known in the art (e.g., spraying, painting, dip coating, vapor depositing, molding, 3-D printing, lithographic techniques (e.g., photolithograph), micro- and nano-pipetting printing techniques, etc.). The mixture can then be cured under high temperature (e.g., at about 50-150° C.). Other suitable curing methods may include ultraviolet or gamma radiation, for example.

Some amount of cross-linking agent can also be included in the mixture to induce cross-linking between polymer molecules. Non-limiting examples of suitable cross-linking agents include isocyanate, carbodiimide, gluteraldehyde or other aldehydes, epoxy, acrylates, free-radical based agents, ethylene glycol diglycidyl ether (EGDE), poly(ethylene glycol) diglycidyl ether (PEGDE), or dicumyl peroxide (DCP). In one embodiment, from about 0.1% to about 15% w/w of cross-linking agent is added relative to the total dry weights of cross-linking agent and polymers added when blending the ingredients (in one example, about 1% to about 10%). During the curing process, substantially all of the cross-linking agent is believed to react, leaving substantially no detectable unreacted cross-linking agent in the final film.

In some embodiments, the bioprotective domain 46 is positioned most distally to the sensing region such that its outer most domain contacts a biological fluid when inserted in vivo. In some embodiments, the bioprotective domain is resistant to cellular attachment, impermeable to cells, and may be composed of a biostable material. While not wishing to be bound by theory, it is believed that when the bioprotective domain 46 is resistant to cellular attachment (for example, attachment by inflammatory cells, such as macrophages, which are therefore kept a sufficient distance from other domains, for example, the enzyme domain), hypochlorite and other oxidizing species are short-lived chemical species in vivo, and biodegradation does not generally occur. Additionally, the materials for forming the bioprotective domain 46 may be resistant to the effects of these oxidative species and have thus been termed biodurable. In some embodiments, the bioprotective domain controls the flux of oxygen and other analytes (for example, glucose) to the underlying enzyme domain (e.g., wherein the functionality of the diffusion resistance domain is built-into the bioprotective domain such that a separate diffusion resistance domain is not required).

In certain embodiments, the thickness of the bioprotective domain may be from about 0.1, 0.5, 1, 2, 4, 6, 8 microns or less to about 10, 15, 20, 30, 40, 50, 75, 100, 125, 150, 175, 200 or 250 microns or more. In some of these embodiments, the thickness of the bioprotective domain may be sometimes from about 1 to about 5 microns, and sometimes from about 2 to about 7 microns. In other embodiments, the bioprotective domain may be from about 20 or 25 microns to about 50, 55, or 60 microns thick. In some embodiments, the glucose sensor may be configured for transcutaneous or short-term subcutaneous implantation, and may have a thickness from about 0.5 microns to about 8 microns, and sometimes from about 4 microns to about 6 microns. In one glucose sensor configured for fluid communication with a host's circulatory system, the thickness may be from about 1.5 microns to about 25 microns, and sometimes from about 3 to about 15 microns. It is also contemplated that in some embodiments, the bioprotective layer or any other layer of the electrode may have a thickness that is consistent, but in other embodiments, the thickness may vary. For example, in some embodiments, the thickness of the bioprotective layer may vary along the longitudinal axis of the electrode end.

Diffusion Resistance Domain

In some embodiments, a diffusion resistance domain 44, also referred to as a diffusion resistance layer, may be used and is situated more proximal to the implantable device relative to the bioprotective domain. In some embodiments, the functionality of the diffusion resistance domain may be built into the bioprotective domain that comprises the surface-active group-containing base polymer. Accordingly, it is to be noted that the description herein of the diffusion resistance domain may also apply to the bioprotective domain. The diffusion resistance domain serves to control the flux of oxygen and other analytes (for example, glucose) to the underlying enzyme domain. As described in more detail elsewhere herein, there exists a molar excess of glucose relative to the amount of oxygen in blood, i.e., for every free oxygen molecule in extracellular fluid, there are typically more than 100 glucose molecules present (see Updike et al., Diabetes Care 5:207-21(1982)). However, an immobilized enzyme-based sensor employing oxygen as cofactor is supplied with oxygen in non-rate-limiting excess in order to respond linearly to changes in glucose concentration, while not responding to changes in oxygen tension. More specifically, when a glucose-monitoring reaction is oxygen-limited, linearity is not achieved above minimal concentrations of glucose. Without a semipermeable membrane situated over the enzyme domain to control the flux of glucose and oxygen, a linear response to glucose levels can be obtained only up to about 40 mg/dL. However, in a clinical setting, a linear response to glucose levels is desirable up to at least about 500 mg/dL.

The diffusion resistance domain 44 includes a semipermeable membrane that controls the flux of oxygen and glucose to the underlying enzyme domain 44, which can optionally render oxygen in non-rate-limiting excess. As a result, the upper limit of linearity of glucose measurement is extended to a much higher value than that which is achieved without the diffusion resistance domain. In some embodiments, the diffusion resistance domain exhibits an oxygen-to-glucose permeability ratio of approximately 200:1, but in other embodiments the oxygen-to-glucose permeability ratio may be approximately 100:1, 125:1, 130:1, 135:1, 150:1, 175:1, 225:1, 250:1, 275:1, 300:1, or 500:1. As a result of the high oxygen-to-glucose permeability ratio, one-dimensional reactant diffusion may provide sufficient excess oxygen at all reasonable glucose and oxygen concentrations found in the subcutaneous matrix (See Rhodes et al., Anal. Chem., 66:1520-1529 (1994)). In some embodiments, a lower ratio of oxygen-to-glucose can be sufficient to provide excess oxygen by using a high oxygen soluble domain (for example, a silicone material) to enhance the supply/transport of oxygen to the enzyme membrane or electroactive surfaces. By enhancing the oxygen supply through the use of a silicone composition, for example, glucose concentration can be less of a limiting factor. In other words, if more oxygen is supplied to the enzyme or electroactive surfaces, then more glucose can also be supplied to the enzyme without creating an oxygen rate-limiting excess.

In some embodiments, the diffusion resistance domain is formed of a base polymer synthesized to include a polyurethane membrane with both hydrophilic and hydrophobic regions to control the diffusion of glucose and oxygen to an analyte sensor. A suitable hydrophobic polymer component may be a polyurethane or polyether urethane urea. Polyurethane is a polymer produced by the condensation reaction of a diisocyanate and a difunctional hydroxyl-containing material. A polyurea is a polymer produced by the condensation reaction of a diisocyanate and a difunctional amine-containing material. Diisocyanates include aliphatic diisocyanates containing from about 4 to about 8 methylene units. Diisocyanates containing cycloaliphatic moieties can also be useful in the preparation of the polymer and copolymer components of the membranes of various embodiments. The material that forms the basis of the hydrophobic matrix of the diffusion resistance domain can be any of those known in the art as appropriate for use as membranes in sensor devices and as having sufficient permeability to allow relevant compounds to pass through it, for example, to allow an oxygen molecule to pass through the membrane from the sample under examination in order to reach the active enzyme or electrochemical electrodes. Examples of materials which can be used to make non-polyurethane type membranes include vinyl polymers, polyethers, polyesters, polyamides, inorganic polymers such as polysiloxanes and polycarbosiloxanes, natural polymers such as cellulosic and protein based materials, and mixtures or combinations thereof.

In one embodiment of a polyurethane-based resistance domain, the hydrophilic polymer component is polyethylene oxide. For example, one useful hydrophilic copolymer component is a polyurethane polymer that includes about 20% hydrophilic polyethylene oxide. The polyethylene oxide portions of the copolymer are thermodynamically driven to separate from the hydrophobic portions of the copolymer and the hydrophobic polymer component. The 20% polyethylene oxide-based soft segment portion of the copolymer used to form the final blend affects the water pick-up and subsequent glucose permeability of the membrane.

Alternatively, in some embodiments, the resistance domain may comprise a combination of a base polymer (e.g., polyurethane) and one or more hydrophilic polymers (e.g., PVA, PEG, polyacrylamide, acetates, PEO, PEA, PVP, and variations thereof). It is contemplated that any of a variety of combination of polymers may be used to yield a blend with desired glucose, oxygen, and interference permeability properties. For example, in some embodiments, the resistance domain may be formed from a blend of a silicone polycarbonate-urethane base polymer and a PVP hydrophilic polymer, but in other embodiments, a blend of a polyurethane, or another base polymer, and one or more hydrophilic polymers may be used instead. In some of the embodiments involving the use of PVP, the PVP portion of the polymer blend may comprise from about 5% to about 50% by weight of the polymer blend, sometimes from about 15% to 20%, and other times from about 25% to 40%. It is contemplated that PVP of various molecular weights may be used. For example, in some embodiments, the molecular weight of the PVP used may be from about 25,000 daltons to about 5,000,000 daltons, sometimes from about 50,000 daltons to about 2,000,000 daltons, and other times from 6,000,000 daltons to about 10,000,000 daltons.

In some embodiments, the diffusion resistance domain 44 can be formed as a unitary structure with the bioprotective domain 46; that is, the inherent properties of the diffusion resistance domain 44 are incorporated into bioprotective domain 46 such that the bioprotective domain 46 functions as a diffusion resistance domain 44.

In certain embodiments, the thickness of the resistance domain may be from about 0.05 microns or less to about 200 microns or more. In some of these embodiments, the thickness of the resistance domain may be from about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 6, 8 microns to about 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 19.5, 20, 30, 40, 50, 60, 70, 75, 80, 85, 90, 95, or 100 microns. In some embodiments, the thickness of the resistance domain is from about 2, 2.5, or 3 microns to about 3.5, 4, 4.5, or 5 microns in the case of a transcutaneously implanted sensor or from about 20 or 25 microns to about 40 or 50 microns in the case of a wholly implanted sensor.

Enzyme Domain

In some embodiments, an enzyme domain 42, also referred to as the enzyme layer, may be used and is situated less distal from the electrochemically reactive surfaces than the diffusion resistance domain 44. The enzyme domain comprises a catalyst configured to react with an analyte. In one embodiment, the enzyme domain is an immobilized enzyme domain 42 including glucose oxidase. In other embodiments, the enzyme domain 42 can be impregnated with other oxidases, for example, galactose oxidase, cholesterol oxidase, amino acid oxidase, alcohol oxidase, lactate oxidase, or uricase. For example, for an enzyme-based electrochemical glucose sensor to perform well, the sensor's response should neither be limited by enzyme activity nor cofactor concentration.

In some embodiments, the catalyst (enzyme) can be impregnated or otherwise immobilized into the bioprotective or diffusion resistance domain such that a separate enzyme domain 42 is not required (e.g., wherein a unitary domain is provided including the functionality of the bioprotective domain, diffusion resistance domain, and enzyme domain). In some embodiments, the enzyme domain 42 is formed from a polyurethane, for example, aqueous dispersions of colloidal polyurethane polymers including the enzyme.

In some embodiments, the thickness of the enzyme domain may be from about 0.01, 0.05, 0.6, 0.7, or 0.8 microns to about 1, 1.2, 1.4, 1.5, 1.6, 1.8, 2, 2.1, 2.2, 2.5, 3, 4, 5, 10, 20, 30 40, 50, 60, 70, 80, 90, or 100 microns. In some embodiments, the thickness of the enzyme domain is between about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1, 1.5, 2, 2.5, 3, 4, or 5 microns and 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 19.5, 20, 25, or microns. In some embodiments, the thickness of the enzyme domain is from about 2, 2.5, or 3 microns to about 3.5, 4, 4.5, or 5 microns in the case of a transcutaneously implanted sensor or from about 6, 7, or 8 microns to about 9, 10, 11, or 12 microns in the case of a wholly implanted sensor.

Electrode Domain

It is contemplated that in some embodiments, such as the embodiment illustrated in FIG. 2C, an optional electrode domain 36, also referred to as the electrode layer, may be provided, in addition to the bioprotective domain and the enzyme domain; however, in other embodiments, the functionality of the electrode domain may be incorporated into the bioprotective domain so as to provide a unitary domain that includes the functionality of the bioprotective domain, diffusion resistance domain, enzyme domain, and electrode domain. In some embodiments, the electrode domain may be replaced by an interference domain. In other embodiments, however, the membrane can include both an interference domain and an electrode domain.

In some embodiments, the electrode domain is located most proximal to the electrochemically reactive surfaces. To facilitate electrochemical reaction, the electrode domain may include a semipermeable coating that maintains hydrophilicity at the electrochemically reactive surfaces of the sensor interface. The electrode domain can enhance the stability of an adjacent domain by protecting and supporting the material that makes up the adjacent domain. The electrode domain may also assist in stabilizing the operation of the device by overcoming electrode start-up problems and drifting problems caused by inadequate electrolyte. The buffered electrolyte solution contained in the electrode domain may also protect against pH-mediated damage that can result from the formation of a large pH gradient between the substantially hydrophobic interference domain and the electrodes due to the electrochemical activity of the electrodes.

In some embodiments, the electrode domain includes a flexible, water-swellable, substantially solid gel-like film (e.g., a hydrogel) having a ‘dry film’ thickness of from about 0.05 microns to about 100 microns, and sometimes from about 0.05, 0.1, 0.15, 0.2, 0.25 0.3, 0.35, 0.4, 0.45, 0.5, 1 microns to about 1.5, 2, 2.5, 3, or 3.5, 4, 4.5, 5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5, 11, 11.5, 12, 13, 14, 15, 16, 17, 18, 19, 19.5, 20, 30, 40, 50, 60, 70, 80, 90, or 100 microns. In some embodiments, the thickness of the electrode domain may be from about 2, 2.5, or 3 microns to about 3.5, 4, 4.5, or 5 microns in the case of a transcutaneously implanted sensor, or from about 6, 7, or 8 microns to about 9, 10, 11, or 12 microns in the case of a wholly implanted sensor. The term ‘dry film thickness’ as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and is not to be limited to a special or customized meaning), and refers without limitation to the thickness of a cured film cast from a coating formulation onto the surface of the membrane by standard coating techniques. The coating formulation may comprise a premix of film-forming polymers and a cross-linking agent and may be curable upon the application of moderate heat.

In certain embodiments, the electrode domain may be formed of a curable mixture of a urethane polymer and a hydrophilic polymer. In some of these embodiments, coatings are formed of a polyurethane polymer having anionic carboxylate functional groups and non-ionic hydrophilic polyether segments, which are cross-linked in the presence of polyvinylpyrrolidone and cured at a moderate temperature of about 50° C.

Particularly suitable for this purpose are aqueous dispersions of fully-reacted colloidal polyurethane polymers having cross-linkable carboxyl functionality (e.g., BAYBOND®; Mobay Corporation). These polymers are supplied in dispersion grades having a polycarbonate-polyurethane backbone containing carboxylate groups identified as W-121 and W-123; and a polyester-polyurethane backbone containing carboxylate groups, identified as W-110-2. In some embodiments, BAYBOND® 123, an aqueous anionic dispersion of an aliphatic polycarbonate urethane polymer sold as a 35 weight percent solution in water and co-solvent N-methyl-2-pyrrolidone, may be used.

In some embodiments, the electrode domain is formed from a hydrophilic polymer that renders the electrode domain substantially more hydrophilic than an overlying domain (e.g., interference domain, enzyme domain). Such hydrophilic polymers may include, a polyamide, a polylactone, a polyimide, a polylactam, a functionalized polyamide, a functionalized polylactone, a functionalized polyimide, a functionalized polylactam or combinations thereof, for example.

In some embodiments, the electrode domain is formed primarily from a hydrophilic polymer, and in some of these embodiments, the electrode domain is formed substantially from PVP. PVP is a hydrophilic water-soluble polymer and is available commercially in a range of viscosity grades and average molecular weights ranging from about 18,000 to about 500,000, under the PVP K® homopolymer series by BASF Wyandotte and by GAF Corporation. In certain embodiments, a PVP homopolymer having an average molecular weight of about 360,000 identified as PVP-K90 (BASF Wyandotte) may be used to form the electrode domain. Also suitable are hydrophilic, film-forming copolymers of N-vinylpyrrolidone, such as a copolymer of N-vinylpyrrolidone and vinyl acetate, a copolymer of N-vinylpyrrolidone, ethylmethacrylate and methacrylic acid monomers, and the like.

In certain embodiments, the electrode domain is formed entirely from a hydrophilic polymer. Useful hydrophilic polymers contemplated include, but are not limited to, poly-N-vinylpyrrolidone, poly-N-vinyl-2-piperidone, poly-N-vinyl-2-caprolactam, poly-N-vinyl-3-methyl-2-caprolactam, poly-N-vinyl-3-methyl-2-piperidone, poly-N-vinyl-4-methyl-2-piperidone, poly-N-vinyl-4-methyl-2-caprolactam, poly-N-vinyl-3-ethyl-2-pyrrolidone, poly-N-vinyl-4,5-dimethyl-2-pyrrolidone, polyvinylimidazole, poly-N,N-dimethylacrylamide, polyvinyl alcohol, polyacrylic acid, polyethylene oxide, poly-2-ethyl-oxazoline, copolymers thereof and mixtures thereof. A blend of two or more hydrophilic polymers may be used in some embodiments.

It is contemplated that in certain embodiments, the hydrophilic polymer used may not be cross-linked, but in other embodiments, cross-linking may be used and achieved by any of a variety of methods, for example, by adding a cross-linking agent. In some embodiments, a polyurethane polymer may be cross-linked in the presence of PVP by preparing a premix of the polymers and adding a cross-linking agent just prior to the production of the membrane. Suitable cross-linking agents contemplated include, but are not limited to, carbodiimides (e.g., 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride, UCARLNK®. XL-25 (Union Carbide)), epoxides and melamine/formaldehyde resins. Alternatively, it is also contemplated that cross-linking may be achieved by irradiation at a wavelength sufficient to promote cross-linking between the hydrophilic polymer molecules, which is believed to create a more tortuous diffusion path through the domain.

The flexibility and hardness of the coating can be varied as desired by varying the dry weight solids of the components in the coating formulation. The term ‘dry weight solids’ as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and is not to be limited to a special or customized meaning), and refers without limitation to the dry weight percent based on the total coating composition after the time the cross-linker is included. In one embodiment, a coating formulation can contain about 6 to about 20 dry weight percent, such as about 8 dry weight percent, PVP; about 3 to about 10 dry weight percent, sometimes about 5 dry weight percent cross-linking agent; and about 70 to about 91 weight percent, sometimes about 87 weight percent of a polyurethane polymer, such as a polycarbonate-polyurethane polymer, for example. The reaction product of such a coating formulation is referred to herein as a water-swellable cross-linked matrix of polyurethane and PVP.

In some embodiments, underlying the electrode domain is an electrolyte phase that when hydrated is a free-fluid phase including a solution containing at least one compound, typically a soluble chloride salt, which conducts electric current. In one embodiment wherein the membrane system is used with a glucose sensor such as is described herein, the electrolyte phase flows over the electrodes and is in contact with the electrode domain. It is contemplated that certain embodiments may use any suitable electrolyte solution, including standard, commercially available solutions. Generally, the electrolyte phase can have the same osmotic pressure or a lower osmotic pressure than the sample being analyzed. In some embodiments, the electrolyte phase comprises normal saline.

Bioactive Agents

It is contemplated that any of a variety of bioactive (therapeutic) agents can be used with the analyte sensor systems described herein, such as the analyte sensor system shown in FIG. 1 . In some embodiments, the bioactive agent is an anticoagulant. The term ‘anticoagulant’ as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and is not to be limited to a special or customized meaning), and refers without limitation to a substance the prevents coagulation (e.g., minimizes, reduces, or stops clotting of blood). In these embodiments, the anticoagulant included in the analyte sensor system may prevent coagulation within or on the sensor. Suitable anticoagulants for incorporation into the sensor system include, but are not limited to, vitamin K antagonists (e.g., Acenocoumarol, Clorindione, Dicumarol (Dicoumarol), Diphenadione, Ethyl biscoumacetate, Phenprocoumon, Phenindione, Tioclomarol, or Warfarin), heparin group anticoagulants (e.g., Platelet aggregation inhibitors: Antithrombin III, Bemiparin, Dalteparin, Danaparoid, Enoxaparin, Heparin, Nadroparin, Parnaparin, Reviparin, Sulodexide, Tinzaparin), other platelet aggregation inhibitors (e.g., Abciximab, Acetylsalicylic acid (Aspirin), Aloxiprin, Beraprost, Ditazole, Carbasalate calcium, Cloricromen, Clopidogrel, Dipyridamole, Epoprostenol, Eptifibatide, Indobufen, Iloprost, Picotamide, Ticlopidine, Tirofiban, Treprostinil, Triflusal), enzymes (e.g., Alteplase, Ancrod, Anistreplase, Brinase, Drotrecogin alfa, Fibrinolysin, Protein C, Reteplase, Saruplase, Streptokinase, Tenecteplase, Urokinase), direct thrombin inhibitors (e.g., Argatroban, Bivalirudin, Desirudin, Lepirudin, Melagatran, Ximelagatran, other antithrombotics (e.g., Dabigatran, Defibrotide, Dermatan sulfate, Fondaparinux, Rivaroxaban), and the like.

In one embodiment, heparin is incorporated into the analyte sensor system, for example by dipping or spraying. While not wishing to be bound by theory, it is believed that heparin coated on the catheter or sensor may prevent aggregation and clotting of blood on the analyte sensor system, thereby preventing thromboembolization (e.g., prevention of blood flow by the thrombus or clot) or subsequent complications. In another embodiment, an antimicrobial is coated on the catheter (inner or outer diameter) or sensor.

In some embodiments, an antimicrobial agent may be incorporated into the analyte sensor system. The antimicrobial agents contemplated may include, but are not limited to, antibiotics, antiseptics, disinfectants and synthetic moieties, and combinations thereof, and other agents that are soluble in organic solvents such as alcohols, ketones, ethers, aldehydes, acetonitrile, acetic acid, methylene chloride and chloroform. The amount of each antimicrobial agent used to impregnate the medical device varies to some extent, but is at least of an effective concentration to inhibit the growth of bacterial and fungal organisms, such as staphylococci, gram-positive bacteria, gram-negative bacilli and Candida.

In some embodiments, an antibiotic may be incorporated into the analyte sensor system. Classes of antibiotics that can be used include tetracyclines (e.g., minocycline), rifamycins (e.g., rifampin), macrolides (e.g., erythromycin), penicillins (e.g., nafcillin), cephalosporins (e.g., cefazolin), other beta-lactam antibiotics (e.g., imipenem, aztreonam), aminoglycosides (e.g., gentamicin), chloramphenicol, sufonamides (e.g., sulfamethoxazole), glycopeptides (e.g., vancomycin), quinolones (e.g., ciprofloxacin), fusidic acid, trimethoprim, metronidazole, clindamycin, mupirocin, polyenes (e.g., amphotericin B), azoles (e.g., fluconazole), and beta-lactam inhibitors (e.g., sulbactam).

Examples of specific antibiotics that can be used include minocycline, rifampin, erythromycin, nafcillin, cefazolin, imipenem, aztreonam, gentamicin, sulfamethoxazole, vancomycin, ciprofloxacin, trimethoprim, metronidazole, clindamycin, teicoplanin, mupirocin, azithromycin, clarithromycin, ofloxacin, lomefloxacin, norfloxacin, nalidixic acid, sparfloxacin, pefloxacin, amifloxacin, enoxacin, fleroxacin, temafloxacin, tosufloxacin, clinafloxacin, sulbactam, clavulanic acid, amphotericin B, fluconazole, itraconazole, ketoconazole, and nystatin.

In some embodiments, an antiseptic or disinfectant may be incorporated into the analyte sensor system. Examples of antiseptics and disinfectants are hexachlorophene, cationic bisiguanides (e.g., chlorhexidine, cyclohexidine) iodine and iodophores (e.g., povidoneiodine), para-chloro-meta-xylenol, triclosan, furan medical preparations (e.g., nitrofurantoin, nitrofurazone), methenamine, aldehydes (glutaraldehyde, formaldehyde) and alcohols. Other examples of antiseptics and disinfectants will readily suggest themselves to those of ordinary skill in the art.

In some embodiments, an anti-barrier cell agent may be incorporated into the analyte sensor system. Anti-barrier cell agents may include compounds exhibiting effects on macrophages and foreign body giant cells (FBGCs). It is believed that anti-barrier cell agents prevent closure of the barrier to solute transport presented by macrophages and FBGCs at the device-tissue interface during FBC maturation. Anti-barrier cell agents may provide anti-inflammatory or immunosuppressive mechanisms that affect the wound healing process, for example, healing of the wound created by the incision into which an implantable device is inserted. Cyclosporine, which stimulates very high levels of neovascularization around biomaterials, can be incorporated into a bioprotective membrane of one embodiment (see U.S. Pat. No. 5,569,462 to Martinson et al.). Alternatively, Dexamethasone, which abates the intensity of the FBC response at the tissue-device interface, can be incorporated into a bioprotective membrane of one embodiment. Alternatively, Rapamycin, which is a potent specific inhibitor of some macrophage inflammatory functions, can be incorporated into a bioprotective membrane of one embodiment.

In some embodiments, an, anti-inflammatory agent may be incorporated into the analyte sensor system to reduce acute or chronic inflammation adjacent to the implant or to decrease the formation of a FBC capsule to reduce or prevent barrier cell layer formation, for example. Suitable anti-inflammatory agents include but are not limited to, for example, nonsteroidal anti-inflammatory drugs (NSAIDs) such as acetometaphen, aminosalicylic acid, aspirin, celecoxib, choline magnesium trisalicylate, diclofenac potassium, diclofenac sodium, diflunisal, etodolac, fenoprofen, flurbiprofen, ibuprofen, indomethacin, interleukin (IL)-10, IL-6 mutein, anti-IL-6 iNOS inhibitors (for example, L-NAME or L-NMDA), Interferon, ketoprofen, acetominophen, ketorolac, leflunomide, melenamic acid, mycophenolic acid, mizoribine, nabumetone, naproxen, naproxen sodium, oxaprozin, piroxicam, rofecoxib, salsalate, sulindac, and tolmetin; and corticosteroids such as cortisone, hydrocortisone, methylprednisolone, prednisone, prednisolone, betamethesone, beclomethasone dipropionate, budesonide, dexamethasone sodium phosphate, flunisolide, fluticasone propionate, paclitaxel, tacrolimus, tranilast, triamcinolone acetonide, betamethasone, fluocinolone, fluocinonide, betamethasone dipropionate, betamethasone valerate, desonide, desoximetasone, fluocinolone, triamcinolone, triamcinolone acetonide, clobetasol propionate, and dexamethasone.

In some embodiments, an immunosuppressive or immunomodulatory agent may be incorporated into the analyte sensor system in order to interfere directly with several key mechanisms necessary for involvement of different cellular elements in the inflammatory response. Suitable immunosuppressive and immunomodulatory agents include, but are not limited to, anti-proliferative, cell-cycle inhibitors, (for example, paclitaxel, cytochalasin D, infiximab), taxol, actinomycin, mitomycin, thospromote VEGF, estradiols, NO donors, QP-2, tacrolimus, tranilast, actinomycin, everolimus, methothrexate, mycophenolic acid, angiopeptin, vincristing, mitomycine, statins, C MYC antisense, sirolimus (and analogs), RestenASE, 2-chloro-deoxyadenosine, PCNA Ribozyme, batimstat, prolyl hydroxylase inhibitors, PPARγ ligands (for example troglitazone, rosiglitazone, pioglitazone), halofuginone, C-proteinase inhibitors, probucol, BCP671, EPC antibodies, catchins, glycating agents, endothelin inhibitors (for example, Ambrisentan, Tesosentan, Bosentan), Statins (for example, Cerivasttin), E. coli heat-labile enterotoxin, and advanced coatings.

In some embodiments, an anti-infective agent may be incorporated into the analyte sensor system. In general, anti-infective agents are substances capable of acting against infection by inhibiting the spread of an infectious agent or by killing the infectious agent outright, which can serve to reduce an immuno-response without an inflammatory response at the implant site, for example. Anti-infective agents include, but are not limited to, anthelmintics (e.g., mebendazole), antibiotics (e.g., aminoclycosides, gentamicin, neomycin, tobramycin), antifungal antibiotics (e.g., amphotericin b, fluconazole, griseofulvin, itraconazole, ketoconazole, nystatin, micatin, tolnaftate), cephalosporins (e.g., cefaclor, cefazolin, cefotaxime, ceftazidime, ceftriaxone, cefuroxime, cephalexin), beta-lactam antibiotics (e.g., cefotetan, meropenem), chloramphenicol, macrolides (e.g., azithromycin, clarithromycin, erythromycin), penicillins (e.g., penicillin G sodium salt, amoxicillin, ampicillin, dicloxacillin, nafcillin, piperacillin, ticarcillin), tetracyclines (e.g., doxycycline, minocycline, tetracycline), bacitracin, clindamycin, colistimethate sodium, polymyxin b sulfate, vancomycin, antivirals (e.g., acyclovir, amantadine, didanosine, efavirenz, foscarnet, ganciclovir, indinavir, lamivudine, nelfinavir, ritonavir, saquinavir, silver, stavudine, valacyclovir, valganciclovir, zidovudine), quinolones (e.g., ciprofloxacin, levofloxacin); sulfonamides (e.g., sulfadiazine, sulfisoxazole), sulfones (e.g., dapsone), furazolidone, metronidazole, pentamidine, sulfanilamidum crystallinum, gatifloxacin, and sulfamethoxazole/trimethoprim.

In some embodiments, a vascularization agent may be incorporated into the analyte sensor system. Vascularization agents generally may include substances with direct or indirect angiogenic properties. In some cases, vascularization agents may additionally affect formation of barrier cells in vivo. By indirect angiogenesis, it is meant that the angiogenesis can be mediated through inflammatory or immune stimulatory pathways. It is not fully known how agents that induce local vascularization indirectly inhibit barrier-cell formation; however, while not wishing to be bound by theory, it is believed that some barrier-cell effects can result indirectly from the effects of vascularization agents.

Vascularization agents may provide mechanisms that promote neovascularization and accelerate wound healing around the membrane or minimize periods of ischemia by increasing vascularization close to the tissue-device interface. Sphingosine-1-Phosphate (SIP), a phospholipid possessing potent angiogenic activity, may be incorporated into the bioprotective membrane. Monobutyrin, a vasodilator and angiogenic lipid product of adipocytes, may also be incorporated into the bioprotective membrane. In another embodiment, an anti-sense molecule (for example, thrombospondin-2 anti-sense), which may increase vascularization, is incorporated into a bioprotective membrane.

Vascularization agents may provide mechanisms that promote inflammation, which is believed to cause accelerated neovascularization and wound healing in vivo. In one embodiment, a xenogenic carrier, for example, bovine collagen, which by its foreign nature invokes an immune response, stimulates neovascularization, and is incorporated into a bioprotective membrane of some embodiments. In another embodiment, Lipopolysaccharide, an immunostimulant, may be incorporated into a bioprotective membrane. In another embodiment, a protein, for example, a bone morphogenetic protein (BMP), which is known to modulate bone healing in tissue, may be incorporated into the bioprotective membrane.

In some embodiments, an angiogenic agent may be incorporated into the analyte sensor system. Angiogenic agents are substances capable of stimulating neovascularization, which can accelerate and sustain the development of a vascularized tissue bed at the tissue-device interface, for example. Angiogenic agents include, but are not limited to, Basic Fibroblast Growth Factor (bFGF), (also known as Heparin Binding Growth Factor-II and Fibroblast Growth Factor II), Acidic Fibroblast Growth Factor (aFGF), (also known as Heparin Binding Growth Factor-I and Fibroblast Growth Factor-I), Vascular Endothelial Growth Factor (VEGF), Platelet Derived Endothelial Cell Growth Factor BB (PDEGF-BB), Angiopoietin-1, Transforming Growth Factor Beta (TGF-β), Transforming Growth Factor Alpha (TGF-Alpha), Hepatocyte Growth Factor, Tumor Necrosis Factor-Alpha (TNFα), Placental Growth Factor (PLGF), Angiogenin, Interleukin-8 (IL-8), Hypoxia Inducible Factor-I (HIF-1), Angiotensin-Converting Enzyme (ACE) Inhibitor Quinaprilat, Angiotropin, Thrombospondin, Peptide KGHK, Low Oxygen Tension, Lactic Acid, Insulin, Copper Sulphate, Estradiol, prostaglandins, cox inhibitors, endothelial cell binding agents (for example, decorin or vimentin), glenipin, hydrogen peroxide, nicotine, and Growth Hormone.

In some embodiments, a pro-inflammatory agent may be incorporated into the analyte sensor system. Pro-inflammatory agents are generally substances capable of stimulating an immune response in host tissue, which can accelerate or sustain formation of a mature vascularized tissue bed. For example, pro-inflammatory agents are generally irritants or other substances that induce chronic inflammation and chronic granular response at the wound-site. While not wishing to be bound by theory, it is believed that formation of high tissue granulation induces blood vessels, which supply an adequate or rich supply of analytes to the device-tissue interface. Pro-inflammatory agents include, but are not limited to, xenogenic carriers, Lipopolysaccharides, S. aureus peptidoglycan, and proteins.

These bioactive agents can be used alone or in combination. The bioactive agents can be dispersed throughout the material of the sensor, for example, incorporated into at least a portion of the membrane system, or incorporated into the device (e.g., housing) and adapted to diffuse through the membrane.

There are a variety of systems and methods by which a bioactive agent may be incorporated into the sensor membrane. In some embodiments, the bioactive agent may be incorporated at the time of manufacture of the membrane system. For example, the bioactive agent can be blended prior to curing the membrane system, or subsequent to membrane system manufacture, for example, by coating, imbibing, solvent-casting, or sorption of the bioactive agent into the membrane system. Although in some embodiments the bioactive agent is incorporated into the membrane system, in other embodiments the bioactive agent can be administered concurrently with, prior to, or after insertion of the device in vivo, for example, by oral administration, or locally, by subcutaneous injection near the implantation site. A combination of bioactive agent incorporated in the membrane system and bioactive agent administration locally or systemically can be used in certain embodiments.

In general, a bioactive agent can be incorporated into the membrane system, or incorporated into the device and adapted to diffuse therefrom, in order to modify the in vivo response of the host to the membrane. In some embodiments, the bioactive agent may be incorporated only into a portion of the membrane system adjacent to the sensing region of the device, over the entire surface of the device except over the sensing region, or any combination thereof, which can be helpful in controlling different mechanisms or stages of in vivo response (e.g., thrombus formation). In some alternative embodiments however, the bioactive agent may be incorporated into the device proximal to the membrane system, such that the bioactive agent diffuses through the membrane system to the host circulatory system.

The bioactive agent can include a carrier matrix, wherein the matrix includes one or more of collagen, a particulate matrix, a resorbable or non-resorbable matrix, a controlled-release matrix, or a gel. In some embodiments, the carrier matrix includes a reservoir, wherein a bioactive agent is encapsulated within a microcapsule. The carrier matrix can include a system in which a bioactive agent is physically entrapped within a polymer network. In some embodiments, the bioactive agent is cross-linked with the membrane system, while in others the bioactive agent is sorbed into the membrane system, for example, by adsorption, absorption, or imbibing. The bioactive agent can be deposited in or on the membrane system, for example, by coating, filling, or solvent casting. In certain embodiments, ionic and nonionic surfactants, detergents, micelles, emulsifiers, demulsifiers, stabilizers, aqueous and oleaginous carriers, solvents, preservatives, antioxidants, or buffering agents are used to incorporate the bioactive agent into the membrane system. The bioactive agent can be incorporated into a polymer using techniques such as described above, and the polymer can be used to form the membrane system, coatings on the membrane system, portions of the membrane system, or any portion of the sensor system.

The membrane system can be manufactured using techniques known in the art. The bioactive agent can be sorbed into the membrane system, for example, by soaking the membrane system for a length of time (for example, from about an hour or less to about a week, or from about 4, 8, 12, 16, or 20 hours to about 1, 2, 3, 4, 5, or 7 days).

The bioactive agent can be blended into uncured polymer prior to forming the membrane system. The membrane system is then cured and the bioactive agent thereby cross-linked or encapsulated within the polymer that forms the membrane system.

In yet another embodiment, microspheres are used to encapsulate the bioactive agent. The microspheres can be formed of biodegradable polymers, including synthetic polymers or natural polymers such as proteins and polysaccharides. As used herein, the term polymer is used to refer to both to synthetic polymers and proteins. U.S. Pat. No. 6,281,015 discloses some systems and methods that can be used in conjunction with the disclosed embodiments. In general, bioactive agents can be incorporated in (1) the polymer matrix forming the microspheres, (2) microparticle(s) surrounded by the polymer which forms the microspheres, (3) a polymer core within a protein microsphere, (4) a polymer coating around a polymer microsphere, (5) mixed in with microspheres aggregated into a larger form, or (6) a combination thereof. Bioactive agents can be incorporated as particulates or by co-dissolving the factors with the polymer. Stabilizers can be incorporated by addition of the stabilizers to the factor solution prior to formation of the microspheres.

The bioactive agent can be incorporated into a hydrogel and coated or otherwise deposited in or on the membrane system. Some hydrogels suitable for use in various embodiments include cross-linked, hydrophilic, three-dimensional polymer networks that are highly permeable to the bioactive agent and are triggered to release the bioactive agent based on a stimulus.

The bioactive agent can be incorporated into the membrane system by solvent casting, wherein a solution including dissolved bioactive agent is disposed on the surface of the membrane system, after which the solvent is removed to form a coating on the membrane surface.

The bioactive agent can be compounded into a plug of material, which is placed within the device, such as is described in U.S. Pat. Nos. 4,506,680 and 5,282,844. In some embodiments, the plug is disposed beneath a membrane system; in this way, the bioactive agent is controlled by diffusion through the membrane, which provides a mechanism for sustained-release of the bioactive agent in the host.

Release of Bioactive Agents

Numerous variables can affect the pharmacokinetics of bioactive agent release. The bioactive agents of the various embodiments can be optimized for short- or long-term release. In some embodiments, the bioactive agents of the various embodiments are designed to aid or overcome factors associated with short-term effects (e.g., acute inflammation or thrombosis) of sensor insertion. In some embodiments, the bioactive agents of the various embodiments are designed to aid or overcome factors associated with long-term effects, for example, chronic inflammation or build-up of fibrotic tissue or plaque material. In some embodiments, the bioactive agents of the various embodiments combine short- and long-term release to exploit the benefits of both.

As used herein, ‘controlled,’ sustained or ‘extended’ release of the factors can be continuous or discontinuous, linear or non-linear. This can be accomplished using one or more types of polymer compositions, drug loadings, selections of excipients or degradation enhancers, or other modifications, administered alone, in combination or sequentially to produce the desired effect.

Short-term release of the bioactive agent in the various embodiments generally refers to release over a period of from about a few minutes or hours to about 2, 3, 4, 5, 6, or 7 days or more.

Loading of Bioactive Agents

The amount of loading of the bioactive agent into the membrane system can depend upon several factors. For example, the bioactive agent dosage and duration can vary with the intended use of the membrane system, for example, the intended length of use of the device and the like; differences among patients in the effective dose of bioactive agent; location and methods of loading the bioactive agent; and release rates associated with bioactive agents and optionally their carrier matrix. Therefore, one skilled in the art will appreciate the variability in the levels of loading the bioactive agent, for the reasons described above.

In some embodiments, in which the bioactive agent is incorporated into the membrane system without a carrier matrix, the level of loading of the bioactive agent into the membrane system can vary depending upon the nature of the bioactive agent. The level of loading of the bioactive agent can be sufficiently high such that a biological effect (e.g., thrombosis prevention) is observed. Above this threshold, the bioactive agent can be loaded into the membrane system so as to imbibe up to 100% of the solid portions, cover all accessible surfaces of the membrane, or fill up to 100% of the accessible cavity space. Typically, the level of loading (based on the weight of bioactive agent(s), membrane system, and other substances present) is from about 1 ppm or less to about 1000 ppm or more, or from about 2, 3, 4, or 5 ppm up to about 10, 25, 50, 75, 100, 200, 300, 400, 500, 600, 700, 800, or 900 ppm. In certain embodiments, the level of loading can be 1 wt. % or less up to about 50 wt. % or more, such as from about 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, or 20 wt. % up to about 25, 30, 35, 40, or 45 wt. %.

When the bioactive agent is incorporated into the membrane system with a carrier matrix, such as a gel, the gel concentration can be optimized, for example, loaded with one or more test loadings of the bioactive agent. The gel can contain from about 0.1 or less to about 50 wt. % or more of the bioactive agent(s), for example from about 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or 0.9 wt. % to about 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, or 45 wt. % or more bioactive agent(s), or from about 1, 2, or 3 wt. % to about 4 or 5 wt. % of the bioactive agent(s). Substances that are not bioactive can also be incorporated into the matrix.

Referring now to microencapsulated bioactive agents, the release of the agents from these polymeric systems generally occurs by two different mechanisms. The bioactive agent can be released by diffusion through aqueous filled channels generated in the dosage form by the dissolution of the agent or by voids created by the removal of the polymer solvent or a pore forming agent during the original micro-encapsulation. Alternatively, release can be enhanced due to the degradation of the encapsulating polymer. With time, the polymer erodes and generates increased porosity and microstructure within the device. This creates additional pathways for release of the bioactive agent.

In some embodiments, the sensor is designed to be bioinert, e.g., by the use of bioinert materials. Bioinert materials do not substantially cause any response from the host. As a result, cells can live adjacent to the material but do not form a bond with it. Bioinert materials include but are not limited to alumina, zirconia, titanium oxide or other bioinert materials generally used in the ‘catheter/catheterization’ art. While not wishing to be bound by theory, it is believed that inclusion of a bioinert material in or on the sensor can reduce attachment of blood cells or proteins to the sensor, thrombosis or other host reactions to the sensor.

EXAMPLES Example 1: General Preparation of Layered Interference Domains

Layered interference domains were prepared as follows. Nine poly(allylamine hydrochloride) (PAH) dip solutions were prepared by dissolving PAH having a molecular weight of approximately 100,000-200,000 g/mol in water to produce an aqueous solution with a concentration of approximately 50 mM of PAH. Each of the resulting nine solutions was then titrated with acetic acid or ammonium hydroxide to a pH of 10.0, 9.75, 9.5, 9.25, 9.0, 8.5, 8.0, 7.5, and 7.0, respectively.

Nine solutions for rinsing after PAH immersion were prepared by titrating water with ammonium hydroxide until a pH of 10.0, 9.75, 9.5, 9.25, 9.0, 8.5, 8.0, 7.5, and 7.0, respectively, was reached.

Five poly(acrylic acid) (PAA) dip solutions were prepared by dissolving PAA having a molecular weight of approximately 100,000 g/mol in water to produce an aqueous solution with a concentration of approximately 50 mM of PAA. Each of the resulting five solutions was then titrated with acetic acid or ammonium hydroxide to a specified pH of 2.5, 3.0, 4.0, 5.0, and 6.0, respectively.

Five solutions for rinsing after PAA immersion were prepared by titrating water with acetic acid until a pH of 2.5, 3.0, 4.0, 5.0 and 6.0, respectively, was reached.

Various interference domains were prepared by sequentially dipping a bare platinum wire into the PAH dip solution, followed by dipping the wire into a rinse solution having a pH corresponding to that of the PAH dip solution. This was followed by dipping the wire into the PAA dip solution, then into a rinse solution having a pH corresponding to that of the PAA dip solution. Interference domains were prepared with 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, and 14 layers by sequentially dipping the wire into PAH dip/rinse solutions followed by PAA dip/rinse solutions according to the above procedure.

A nine-layered interference domain was prepared by: (1) dipping a bare platinum wire into a PAH dip solution, then dipping the wire into the corresponding rinse solution; (2) dipping the wire into the PAA dip solution, then dipping the wire into the PAA rinse solution; (3) dipping the wire into a PAH dip solution, then dipping the wire into the corresponding rinse solution; (4) dipping the wire into the PAA dip solution, then dipping the wire into the corresponding rinse solution; (5) dipping the wire into a PAH dip solution, then dipping the wire into the corresponding rinse solution; (6) dipping the wire into the PAA dip solution, then dipping the wire into the corresponding rinse solution; (7) dipping the wire into a PAH dip solution, then dipping the wire into the corresponding rinse solution; (8) dipping the wire into the PAA dip solution, then dipping the wire into the corresponding rinse solution; and (9) dipping the wire into a PAH dip solution, then dipping the wire into the corresponding rinse solution. The resulting nine-layered interference domain contained 5 layers of PAH and 4 layers of PAA.

Other interference domains are prepared by dipping a wire into alternating solutions of PAH, PDADMAC, PAA, PSS, and/or PVS. An interference domain with a single layer is prepared, for example, by dipping the wire into a PAH or PAA dip solution.

In alternative embodiments, interference domains can be prepared whereby a polyanionic polymer is first deposited onto a bare wire, for example, by dipping a bare wire into a polyanionic solution (such as PAA), then into a rinse solution, then into a polycationic solution (such as PAH), then into a rinse solution.

Example 2: Preparation of PAH/PVS Interference Domains

Layered interference domains were prepared using poly(allylamine hydrochloride) (PAH) and poly(vinyl sulfate) (PVS) as follows. Dipping solutions of PAH and corresponding rinse solutions were prepared according to Example 1.

Poly(vinyl sulfate) (PVS) dipping solutions and corresponding rinse solutions were prepared in a manner corresponding to the preparation of the PAA solutions described in Example 1.

Various interference domains were prepared by sequentially dipping a bare platinum wire into the PAH dip solution, followed by the corresponding rinse solution, followed by the PVS dip solution, and followed by the corresponding rinse solution. Interference domains were prepared with 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, and 14 layers by sequentially dipping the wire into PAH dip/rinse solutions followed by PVS dip/rinse solutions.

For example, a four-layered interference domain was prepared by: (1) dipping a bare wire into a PAH dip solution, then dipping the wire into the corresponding rinse solution; (2) dipping the wire into the PVS dip solution, then dipping the wire into the corresponding rinse solution; (3) dipping the wire into a PAH dip solution, then dipping the wire into the corresponding rinse solution; and (4) dipping the wire into the PVS dip solution, then dipping the wire into the corresponding rinse solution. The resulting four-layered interference domain contained 2 layers of PAH and 2 layers of PVS.

An interference domain with a single layer of PAH was prepared by dipping the wire into a PAH dip solution.

Example 3: Effect of Different Numbers of PAH/PVS Layers

The number of PAH and PVS layers in interference domains was varied to determine the effect of the number of layers on the permeability of the resulting interference domain to H₂O₂, acetaminophen, ascorbic acid, and uric acid. Sensors were prepared comprising platinum electrodes coated with interference domains prepared according to the procedure described in Example 2. Specifically, an interference domain containing a single layer of PAH was prepared. Also, interference domains containing 2, 3, and 4 layers of alternating PAH and PVS layers were prepared. An enzyme domain was added according to methods known in the art. The resulting sensors were sequentially placed into solutions that contained either H₂O₂, acetaminophen, ascorbic acid, or uric acid. The average current response (pA) for each solution was measured. The sensitivity of the current response versus the concentration of H₂O₂ was determined. The selectivity of the interference domain with respect to H₂O₂ was determined for acetaminophen, ascorbic acid, and uric acid. A sensor containing an enzyme domain, without an interference domain, was tested as a control. The results are shown in Table 1 and FIGS. 5A and 5B. The data demonstrates that the interference domain possessed high sensitivity to H₂O₂ for interference domains having from 1 to 4 layers, with a slight drop-off observed at 3 layers, and the ability to substantially block acetaminophen (2 to 4 layers) and uric acid (1, 2, or 4 layers).

TABLE 1 Sensitivity to H₂O₂ (pA Selectivity - Selectivity - response/ acetamin- ascorbic Selectivity - Sensor μM H₂O₂) ophen acid uric acid Interference 2336 0.59 0.55 0.27 Domain: 1 layer of PAH Interference 2438 0.30 0.53 0.21 Domain: 2 layers PAH/PVS Interference 1844 0.52 0.60 0.30 Domain: 3 layers PAH/PVS Interference 1719 0.27 0.56 0.17 Domain: 4 layers PAH/PVS No interference 2281 0.64 0.58 0.35 domain Note: Selectivity is calculated by dividing the sensitivity to the interferent (e.g., acetaminophen, ascorbic acid, or uric acid) by the H₂O₂ sensitivity.

Example 4: Effect of pH on H₂O₂ and Acetaminophen Permeability

The pH of PAH and PAA dip solutions was varied to determine the effect of pH on the resulting interference domains' permeability to H₂O₂ and acetaminophen. Interference domains were prepared according to the procedure described in Example 1, and deposited upon a platinum wire electrode. The pH of PAH and PAA dipping solutions used to prepare the interference domains, and the number of layers of PAH and PAA in each interference domain are shown in Tables 2 and 3. Each of the interference domains shown in Table 1 were sequentially placed into the following seven solutions: (1) PBS buffer at a pH of 7.3 at 37° C.; (2) 2 μM aqueous H₂O₂; (3) 4 μM aqueous H₂O₂; (4) 6 μM aqueous H₂O₂; (5) 132 μM aqueous acetaminophen; (6) 264 μM aqueous acetaminophen; and (7) 396 μM aqueous acetaminophen. The average current response (pA) of each of the seven solutions was measured. The sensitivity of the current response versus the concentration of H₂O₂ was determined for solutions (2)-(4). The sensitivity of the current response versus the concentration of acetaminophen was determined for solutions (5)-(7). The results are shown in Tables 2 and 3, and in FIGS. 6A and 6B. The data show that PAA solutions of low pH (i.e., less than 3) produce interference layers with diminished response to acetaminophen. The data also show that PAH solutions of high pH (i.e., greater than 8) produce interference layers with diminished response to acetaminophen.

TABLE 2 Sensitivity to Sensitivity to Number H₂O₂ acetaminophen pH of PAH Number of pH of PAA of (pA (pA response/ dipping PAH dipping PAA response/ μM Selectivity- solution layers solution layers μM H₂O₂) acetaminophen acetaminophen 10.0 5 2.5 4 961 7 0.007 10.0 5 3.0 4 768 5 0.007 10.0 5 4.0 4 1346 23 0.017 10.0 5 5.0 4 1609 28 0.017 10.0 5 6.0 4 1664 21 0.013

TABLE 3 Sensitivity to Sensitivity to Number H₂O₂ acetaminophen pH of PAH Number of pH of PAA of (pA (pA response/ dipping PAH dipping PAA response/ μM Selectivity- solution layers solution layers μM H₂O₂) acetaminophen acetaminophen 10.0 5 3.0 4 1199 10 0.0083 9.75 5 3.0 4 1253 11 0.0088 9.5 5 3.0 4 1276 10 0.0078 9.25 5 3.0 4 1292 11 0.0085 9.0 5 3.0 4 1267 12 0.0095 8.5 5 3.0 4 1438 16 0.011 8.0 5 3.0 4 1550 18 0.012 7.5 5 3.0 4 2071 122 0.0589 7.0 5 3.0 4 2222 122 0.0549

Example 5: pH Effect on Acetaminophen Performance

The effect of pH of PAH and PAA dipping solutions on interference domain sensitivity to acetaminophen was determined. Interference domains were prepared according to the procedure described in Example 1 and deposited on a platinum wire. Specifically, interference domains containing 9 total alternating layers of PAH and PAA were prepared. The results are shown in Tables 4 and 5, and in FIGS. 7A and 7B. The results demonstrate a substantial reduction in sensitivity to acetaminophen when a basic pH PAH solution having a pH above 9 is combined with an acidic PAA solution having a pH below 3.

TABLE 4 pH of pH of Sensitivity to PAH Number PAA Number acetaminophen dipping of PAH dipping of PAA (pA response/ solution layers solution layers μM acetaminophen) 9.5 5 4.0 4 41 9.5 5 3.5 4 20 9.5 5 3.25 4 24 9.5 5 3.0 4 22 9.5 5 2.75 4 14 9.5 5 2.5 4 8 9.5 5 2.25 4 11

TABLE 5 pH of pH of Sensitivity to PAH Number PAA Number acetaminophen dipping of PAH dipping of PAA (pA response/ solution layers solution layers μM acetaminophen) 10.0 5 3.0 4 30 9.75 5 3.0 4 20 9.5 5 3.0 4 22 9.5 5 3.0 4 22 9.25 5 3.0 4 25 9.0 5 3.0 4 53

Example 6: Effect of Different Number of Layers

The number of PAH and PAA layers in interference domains was varied to determine the effect of the number of layers on the resulting interference domain's permeability to H₂O₂ and acetaminophen. Various interference domains were prepared according to the procedure described in Example 1 and deposited on a platinum wire. Specifically, interference domains containing 3, 5, 7, and 9 layers of alternating PAH and PAA layers were prepared. Then an enzyme domain was added according to methods known in the art. Each of the sensors were sequentially placed into the following eight solutions: (1) PBS buffer at a pH of 7.3 at 37° C.; (2) 1 μM aqueous H₂O₂; (3) 2 μM aqueous H₂O₂; (4) 3 μM aqueous H₂O₂; (5) PBS buffer at a pH of 7.3 at 37° C.; (6) 10 μM aqueous acetaminophen; (7) 50 μM aqueous acetaminophen; and (8) 100 μM aqueous acetaminophen. The average current response (pA) for each of the eight solutions was measured. The sensitivity of the current response versus the concentration of H₂O₂ was determined for solutions (2)-(4). The sensitivity of the current response versus the concentration of acetaminophen was determined for solutions (6)-(8). A sensor containing a polyurethane electrode domain and an enzyme domain was tested as a control. The results are shown in Table 6, and in FIGS. 8A and 8B. An increase in number of layers resulted in an incremental but acceptably low decrease in sensitivity to H₂O₂, with a disproportionate reduction in sensitivity to acetaminophen. A particularly dramatic decrease in sensitivity to acetaminophen was observed in going from three layers to five layers.

TABLE 6 Selectivity- Sensitivity to acetaminophen Sensitivity to acetaminophen (sensitivity H₂O₂ (pA response/ acetaminophen/ (pA response/ μM sensitivity sensor μM H₂O₂) acetaminophen) H₂O₂) Interference Domain: 2126 101 0.047 3 layers PAH/PAA Interference Domain: 1857 24 0.013 5 layers PAH/PAA Interference Domain: 1482 12 0.008 7 layers PAH/PAA Interference Domain: 1280 8 0.006 9 layers PAH/PAA Interference Domain: 1082 5 0.005 21 layers PAH/PAA Interference Domain: 1137 5 0.004 23 layers PAH/PAA Interference Domain: 965 4 0.004 25 layers PAH/PAA Interference Domain: 1027 5 0.005 27 layers PAH/PAA Interference Domain: 953 4 0.004 29 layers PAH/PAA Interference Domain: 844 4 0.005 31 layers PAH/PAA Control 2593 1587 0.614

Example 7

The effect of first depositing a cationic layer versus first depositing an anionic layer on sensitivity and selectivity of the resulting interference domain was determined. Interference domains were prepared according to the procedure described in Example 1 and deposited on platinum wire.

Specifically, a first interference domain containing 9 total alternating layers of PAH and PAA was prepared. The first layer deposited on the platinum wire for first interference domain was the cationic polymer PAH. Therefore, the first interference domain contained the following layers: Layer 1=PAH, Layer 2=PAA, Layer 3=PAH, Layer 4=PAA, Layer 5=PAH, Layer 6=PAA, Layer 7=PAH, Layer 8=PAA, Layer 9=PAH.

A second interference domain containing 9 total alternating layers of PAA and PAH was prepared. The first layer deposited on the platinum wire for the second interference domain was the anionic polymer PAA. Therefore, the second interference domain contained the following layers: Layer 1=PAA, Layer 2=PAH, Layer 3=PAA, Layer 4=PAH, Layer 5=PAA, Layer 6=PAH, Layer 7=PAA, Layer 8=PAH, Layer 9=PAA.

The sensitivity to H₂O₂, sensitivity to acetaminophen, and selectivity were determined for both interference domains with and without an added enzyme domain. The results are shown in Table 7.

TABLE 7 Selectivity- Sensitivity to acetaminophen Sensitivity to acetaminophen (sensitivity H₂O₂ (pA response/ acetaminophen/ (pA response/ μM sensitivity ) sensor μM H₂O₂) acetaminophen) H₂O₂ First Interference 2.875 20 0.007 Domain (PAH layer first) First Interference 2.374 27 0.012 Domain (PAH layer first) + Enzyme Domain Second Interference 4.469 34 0.080 Domain (PAA layer first Second Interference 1.733 52 0.030 Domain (PAA layer first) + Enzyme Domain

Example 8: Preparation of PAH/PSS Interference Domains

Layered interference domains are prepared using poly(allylamine hydrochloride) (PAH) and poly(styrene sulfate) (PSS) as follows. PAH dipping and rinse solutions are prepared according to Example 1.

Poly(styrene sulfate) (PSS) dipping and rinse solutions are prepared in a manner to the preparation of the PAA solutions described in Example 1.

Interference domains are prepared by sequentially dipping a bare wire, e.g.,, a platinum wire, into the PAH dip solution, followed by the corresponding rinse solution, followed by the PSS dip solution, and followed by the corresponding rinse solution. Interference domains are prepared with 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, and 14 layers by sequentially dipping the wire into PAH dip/rinse solutions, followed by PSS dip/rinse solutions.

For example, a seven-layered interference domain can be prepared by: (1) dipping a bare wire into a PAH dip solution, then dipping the wire into the corresponding rinse solution; (2) dipping the wire into the PSS dip solution, then dipping the wire into the corresponding rinse solution; (3) dipping the wire into a PAH dip solution, then dipping the wire into the corresponding rinse solution; (4) dipping the wire into the PSS dip solution, then dipping the wire into the corresponding rinse solution; (5) dipping the wire into a PAH dip solution, then dipping the wire into the corresponding rinse solution; (6) dipping the wire into the PSS dip solution, then dipping the wire into the corresponding rinse solution; and (7) dipping the wire into a PAH dip solution, then dipping the wire into the corresponding rinse solution.

The resulting seven-layered interference domain contains 4 layers of PAH and 3 layers of PSS.

Example 9: Preparation of PDADMAC/PAA Interference Domains

Layered interference domains are prepared using poly(diallyldimethylammonium chloride) (PDADMAC) and poly(acrylic acid) (PAA) as follows. PDADMAC dipping and rinse solutions are prepared in a similar manner as the PAH solutions described in Example 1.

PAA dipping and rinse solutions are prepared as described in Example 1.

Various interference domains are prepared by sequentially dipping a bare wire, e.g., a platinum wire, into the PDADMAC dip solution, followed by the corresponding rinse solution, followed by the PAA dip solution, and followed by the corresponding rinse solution. Interference domains are prepared with 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, and 14 layers by sequentially dipping the wire into PDADMAC dip/rinse solutions followed by PAA dip/rinse solutions.

For example, an six-layered interference domain can be prepared by: (1) dipping a bare wire into a PDADMAC dip solution, then dipping the wire into the corresponding rinse solution; (2) dipping the wire into the PAA dip solution, then dipping the wire into the corresponding rinse solution; (3) dipping the wire into a PDADMAC dip solution, then dipping the wire into the corresponding rinse solution; (4) dipping the wire into the PAA dip solution, then dipping the wire into the corresponding rinse solution; (5) dipping the wire into a PDADMAC dip solution, then dipping the wire into the corresponding rinse solution; and (6) dipping the wire into the PAA dip solution, then dipping the wire into the corresponding rinse solution.

The resulting six-layered interference domain contains 3 layers of PDADMAC and 3 layers of PSS.

Example 10: Preparation of PDADMAC/PSS Interference Domains

Layered interference domains are prepared using poly(diallyldimethylammonium chloride) (PDADMAC) and poly(styrene sulfonate) (PSS) as follows. PDADMAC dipping and rinse solutions are prepared in a similar manner as the PAH solutions described in Example 1.

PSS dipping and rinse solutions are prepared in a similar manner as the PAA solutions described in Example 1.

Interference domains are prepared by sequentially dipping a bare wire, e.g., a platinum wire, into the PDADMAC dip solution, followed by the corresponding rinse solution, followed by the PSS dip solution, and followed by the corresponding rinse solution. Interference domains are prepared with 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, and 14 layers by sequentially dipping the wire into PDADMAC dip/rinse solutions followed by PSS dip/rinse solutions.

For example, an five-layered interference domain can be prepared by: (1) dipping a bare wire into a PDADMAC dip solution, then dipping the wire into the corresponding rinse solution; (2) dipping the wire into the PSS dip solution, then dipping the wire into the corresponding rinse solution; (3) dipping the wire into a PDADMAC dip solution, then dipping the wire into the corresponding rinse solution; (4) dipping the wire into the PSS dip solution, then dipping the wire into the corresponding rinse solution; and (5) dipping the wire into a PDADMAC dip solution, then dipping the wire into the corresponding rinse solution.

The resulting five-layered interference domain contains 3 layers of PDADMAC and 2 layers of PSS.

Example 11: Preparation of poly(acetylene) Interference Domains

Layered interference domains are prepared using poly(acetylene) as follows. A dipping solution of poly(acetylene) is prepared by dissolving poly(acetylene) in a suitable solvent, such as dichloromethane or as a colloidal suspension.

Interference domains are prepared by sequentially dipping a bare wire, e.g., a platinum wire, into the poly(acetylene) dip solution, followed by a time period where the wire is permitted to dry, followed by dipping the wire into the poly(acetylene) solution, and followed by a time period where the wire is permitted to dry. Interference domains are prepared with 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, and 14 layers by sequentially dipping the wire into the poly(acetylene) dip solution.

For example, a four-layered interference domain can be prepared by: (1) dipping a bare wire into the poly(acetylene) dip solution, then waiting for the wire to dry; (2) dipping the wire into the poly(acetylene) dip solution, then waiting for the wire to dry; (3) dipping the wire into the poly(acetylene) dip solution, then waiting for the wire to dry; and (4) dipping the into the poly(acetylene) dip solution, then waiting for the wire to dry.

Example 12: Preparation of poly(p-phenylene) Interference Domains

Layered interference domains are prepared using poly(p-phenylene) as follows. A dipping solution of poly(p-phenylene) is prepared by dissolving poly(p-phenylene) in a suitable solvent, such as dichloromethane.

Various interference domains are prepared by sequentially dipping a bare wire, e.g., a platinum wire, into the poly(p-phenylene) dip solution, followed by a time period where the wire is permitted to dry, followed by dipping the wire into the poly(p-phenylene) solution, and followed by a time period where the wire is permitted to dry. Interference domains are prepared with 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, and 14 layers by sequentially dipping the wire into the poly(p-phenylene) dip solution.

For example, an three-layered interference domain can be prepared by: (1) dipping a bare wire into the poly(p-phenylene) dip solution, then waiting for the wire to dry; (2) dipping the wire into the poly(p-phenylene) dip solution, then waiting for the wire to dry; and (3) dipping the wire into the poly(p-phenylene) dip solution, then waiting for the wire to dry.

Terms and phrases used in this document, and variations thereof, unless otherwise expressly stated, should be construed as open ended as opposed to limiting. As examples of the foregoing, the term ‘including’ should be read to mean ‘including, without limitation’ or the like; the term ‘comprising’ as used herein is synonymous with ‘including,’ ‘containing,’ or ‘characterized by,’ and is inclusive or open-ended and does not exclude additional, unrecited elements or method steps; the term ‘example’ is used to provide exemplary instances of the item in discussion, not an exhaustive or limiting list thereof; and adjectives such as ‘known’, ‘normal’, ‘standard’, and terms of similar meaning should not be construed as limiting the item described to a given time period or to an item available as of a given time, but instead should be read to encompass known, normal, or standard technologies that may be available or known now or at any time in the future. Likewise, a group of items linked with the conjunction ‘and’ should not be read as requiring that each and every one of those items be present in the grouping, but rather should be read as ‘and/or’ unless expressly stated otherwise. Similarly, a group of items linked with the conjunction ‘or’ should not be read as requiring mutual exclusivity among that group, but rather should be read as ‘and/or’ unless expressly stated otherwise. In addition, as used in this application, the articles ‘a’ and ‘an’ should be construed as referring to one or more than one (i.e., to at least one) of the grammatical objects of the article. By way of example, ‘an element’ means one element or more than one element.

The presence in some instances of broadening words and phrases such as ‘one or more’, ‘at least’, ‘but not limited to’, or other like phrases shall not be read to mean that the narrower case is intended or required in instances where such broadening phrases may be absent.

All numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification are to be understood as being modified in all instances by the term ‘about.’ Accordingly, unless indicated to the contrary, the numerical parameters set forth herein are approximations that may vary depending upon the desired properties sought to be obtained. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of any claims in any application claiming priority to the present application, each numerical parameter should be construed in light of the number of significant digits and ordinary rounding approaches.

Furthermore, although the foregoing has been described in some detail by way of illustrations and examples for purposes of clarity and understanding, it is apparent to those skilled in the art that certain changes and modifications may be practiced. Therefore, the description and examples should not be construed as limiting the scope of the invention to the specific embodiments and examples described herein, but rather to also cover all modification and alternatives coming with the true scope and spirit of the invention. 

What is claimed is:
 1. A method for forming a membrane on an implantable device configured for continuous measurement of an analyte concentration, comprising: applying a first solution to an implantable sensor or a layer deposited thereon, the first solution having a first pH and comprising polycations; drying the first solution to form a polycationic layer; applying a second solution to the polycationic layer, the second solution having a second pH and comprising polyanions; and drying the second solution to form a polyanionic layer.
 2. The method of claim 1, wherein the first pH is approximately a pK_(b) of the polycations.
 3. The method of claim 1, wherein the first pH is from about 9 to
 10. 4. The method of claim 1, wherein the second pH is approximately a pK_(a) of the polyanions.
 5. The method of claim 1, wherein the second pH is from about 2 to
 3. 6. The method of claim 1, further comprising applying a solution comprising an enzyme onto at least one of a polycationic layer or a polyanionic layer to form an enzyme layer.
 7. The method of claim 6, wherein the analyte is glucose, and wherein the enzyme is glucose oxidase.
 8. An implantable device for continuous measurement of glucose concentration, the device comprising: an electrode configured to generate a signal indicative of a concentration of glucose in a host; and a membrane located over the electrode, the membrane comprising: a first domain that comprises an enzyme configured to react with glucose; a second domain configured to reduce passage therethrough of an interferent, the second domain comprising a plurality of alternating polyelectrolyte layers, wherein the plurality of alternating polyelectrolyte layers comprise at least one of a polycationic layer and a polyanionic layer.
 9. The implantable device of claim 8, wherein the second domain comprises an odd number of alternating polyelectrolyte layers.
 10. The imlantable device of claim 8, wherein the second domain comprises a plurality of polycationic layers, and wherein the polycationic layers form the most distal layer and the most proximal layer of the second domain.
 11. The implantable device of claim 8, wherein the second domain comprises at least three alternating polyelectrolyte layers.
 12. The implantable device of claim 11, wherein the first and third most distal layers with respect to the electrode are polycationic layers, and the second most distal layer with respect to the electrode is a polyanionic layer.
 13. The implantable device of claim 8, wherein the second domain comprises at least five alternating polyelectrolyte layers.
 14. The implantable device of claim 13, wherein the first, third, and fifth most distal layers with respect to the electrode are polycationic layers, and the second and fourth most distal layers with respect to the electrode are polyanionic layers.
 15. The imlantable device of claim 8, wherein the second domain comprises at least seven alternating polyelectrolyte layers.
 16. The implantable device of claim 15, wherein the first, third, fifth, and seventh most distal layers with respect to the electrode are polycationic layers, and the second, fourth, and sixth most distal layers with respect to the sensor are polyanionic layers.
 17. The implantable device of claim 8, wherein the first domain is distal to the second domain with respect to the electrode.
 18. The implantable device of claim 8, wherein the electrode comprises an electroactive surface.
 19. The implantable device of claim 18, wherein the second domain contacts the electroactive surface.
 20. The imlantable device of claim 8, wherein the polycationic layer comprises a polycation with an average linear charge density from about 2 to 10 e/Å, and wherein the polyanionic layer comprises a polyanion with an average linear charge density from about 2 to 10 e/Å.
 21. The implantable device of claim 8, wherein the polycationic layer comprises a polycation with an average linear charge density from about 2 to 3 e/Å, and wherein the polyanionic layer comprises a polyanion with an average linear charge density from about 2 to 3 e/Å. 